Soyou've got a Pokmon that you just hatched from an egg and want to get it up to a level where it can hold its own against the Elite Four or the level at which it evolves? Prefer to make your Pokmon do the hard work by itself instead of abusing the EXP Share or the Switch option? Okay, so not a lot of serious people do that, fine, but I do, so basically here is what I find to be the best set of areas to go through with your Pokmon. This is analyzed in terms of the maximum and minimum levels found in the location and the average experience gained per 100 encounters - the suggested locations should have just about the optimal EXP output for what your Pokmon can handle at any given moment.
The levels indicated apply to your Pokmon (rather than the wild ones) and show only my approximations of when it is appropriate to move on to the next stage. Of course it depends heavily on how strong your Pokmon is and what moves it knows, so if it seems things are being a bit too easy or too hard, by all means switch to the next or previous stage as appropriate until you've found a good balance.
If you can be bothered to play Hoenn Sounds before every encounter, the Violet City entrance of Dark Cave is going to net you the most experience here, especially when Dunsparce are swarming. Otherwise, if you're playing SoulSilver or your Pokmon isn't very good at Geodude and Zubat, you're going to be better off on Route 30. It is always better than Dark Cave without Hoenn Sounds or Swarming Dunsparce in SoulSilver, thanks to getting some level 3 Ledyba instead of level 2 Pidgey; in HeartGold they are extremely close and it's really mostly a matter of which Pokmon set you prefer.
Route 33 is better in SoulSilver, but Union Cave 1F in HeartGold. The former also has slightly higher-leveled Pokmon, but if you're playing SoulSilver you can also use Union Cave if you're having difficulties on Route 33. If you need even lower-leveled, Viridian Forest works, especially at night.
As soon as you can migrate to Route 34, do so, because it gives quite a jump. If your Pokmon is weak to Psychic, you may want to consider Rock Tunnel 1F instead, especially if you can be bothered to turn on Hoenn Sounds (since that in fact makes it give considerably more experience than Route 34). If that isn't working out either, you can try Route 25, but that gives considerably less.
In HeartGold, Route 47 (inside cave) is the best thing around this level by far. The moment you can go here, do it. In SoulSilver, however, Ice Path 1F and B1F (or even B2F and B3F if you can take the higher levels and poor accessibility) is actually better, and it's still a fair second choice in HeartGold.
Though not the most accessible location in the world, the Mt. Mortar upper cave gives the most experience here in the morning or day, but at night you should go for the somewhat more accessible Route 27 - it gives even more experience than Mt. Mortar in SoulSilver, and only slightly less in HeartGold. Otherwise, stay where you are in SoulSilver, but go to Route 26 in HeartGold.
To stick with the Seafoam Islands theme, B3F gives still more experience. Though Victory Road actually comes extremely close and is more accessible, so it might be preferable (especially if its Pokmon are more convenient).
Guess. Yeah, it's yet another floor of Seafoam Islands. Of course that's so inaccessible you almost might not want to bother, except that it overshadows the competition to such a great degree that all the time it's going to take to get there will probably be worth it. Regardless, stay on Victory Road if this is completely unacceptable.
Route 28 is your best bet now, by a considerable margin. If not, though Cerulean Cave 2F is certainly no paragon of accessibility, it is still probably somewhat preferable to Seafoam B4F in that regard, and it actually gives a little bit more experience at night.
Mt. Silver (2F) is the highest-experience training spot in the game, though of course not the most accessible place. Once you've outgrown that, the only thing to do is rebattling Gym leaders or the Elite Four.
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This team was made to defeat Lance. I didn't worry about Red, because going from Lance to Red is mostly just boring level grinding regardless of your team. I guess you can use Snorlax instead of Tauros and use post-game moves like bounce and stone edge if you want your grind to be slightly less boring. In addition to the restrictions in the original post, I put these on myself because I think they make the game more fun. You will obviously want a different team if you use different restrictions.
The starter's job is getting you from New Bark Town to Route 43. More specifically, it must defeat Morty, because that is required to surf on Route 43. Chikorita is obviously the worst for the job, as Bayleef's highest attack stat is only 63, and all of its STAB moves get resisted by Li's, Falkner's, Bugsy's, the rival's, and Morty's Pokemon. Feraligatr works great in the late game, because it learns some moves that are super effective against many Pokemon in the last 3 gyms and Pokemon League. However, this team makes the starter pretty useless after reaching Route 43, regardless of how many coverage moves it learns. Before Route 43, Croconaw's STAB moves are significantly weaker than Quilava's. This is why I think Cyndaquil, which learns fire blast from the Department Store, is the best at getting you from New Bark Town to Route 43.
The fire attacks besides fire blast are Cyndaquil's strongest moves before fire blast, and they remain useful when you want an attack that has high accuracy or PP. Fire blast is for Whitney, the rival, and Morty. Dig covers fire and rock and saves a bit of money on escape ropes. Focus blast covers rock and some other things that resist fire. The normal attacks are there because Typhlosion's coverage moves kind of suck, and there are a bunch of Pokemon that resist fire, ground, and fighting.
This team does not need an electric Pokemon for the rival (despite Quilava's type disadvantage) or a fighting Pokemon for Whitney (despite the fact that Whitney is a meme). If you use only Cyndaquil in every battle before Route 43, then it will earn all the experience, and it can easily go above level 27 or 28 before Whitney. At those levels, its charcoal-boosted fire blast will 2HKO Whitney's Miltank and the rival's Croconaw. Cyndaquil's most difficult battle is probably the rival in Azalea Town, because it's before fire blast. Luckily, the dumb programming makes his Croconaw spam scary face, even after it's already faster than your Quilava, so you have plenty of turns to win with your not very effective flame wheel.
This is the Pokemon that you catch on Route 43, not the Lake of Rage. When you surf on this route, it is not possible to find anything between levels 26 and 49, but it is possible to find a level 50 Magikarp. So you can put any Pokemon between levels 26 and 50 in your first slot and use some repels, and it should be pretty easy to find this Magikarp in 3 or 4 minutes. Then use one rare candy, and you have a level 51 Gyarados before the fifth gym. It is very possible that the specific combination of Typhlosion, Gyarados, Heracross, Tauros, Krabby, and Farfetch'd is not the best team in HGSS, but I don't think anyone can seriously argue that any other Pokemon in HGSS is better than this level 51 Gyarados.
Strength is Gyarados's strongest move before waterfall, and it remains useful for covering stuff that resists waterfall. Surf covers stuff that resists strength. Waterfall is Gyarados's strongest STAB move. Dragon dance raises Gyarados's game-breaking physical attack to even more game-breaking amounts. You can use ice fang if you think strength is not doing enough damage against grass and dragon.
Gyarados can solo any trainer battle between Route 43 and Lance using only strength, surf, waterfall, and dragon dance. It doesn't need ice fang, items, or EVs. It doesn't even need to reach level 52. I included the extra unnecessary stuff just to optimize this moveset a little bit. I'd say the most difficult battles (ie. the ones that require the most luck to solo) are Will and Karen. Both of them lead with confuse ray users, so it's dangerous to use more than one dragon dance against them. Both have another Pokemon that can survive any move after one dragon dance and then use hypnosis or stun spore.
Because Typhlosion and Gyarados can handle all of the boss battles, the next 2 Pokemon have 2 jobs: counting toward the 6 Pokemon requirement in the original post, and effortlessly defeating regular trainers. Consequently, I want Pokemon that are not too difficult to catch, can be caught at a high level, do not share types with my other battlers, have a strong attack stat without needing to evolve (or evolves very soon after getting caught), and can learn strong moves.
Strength and return are Tauros's strongest STAB moves. Headbutt can help you save PP while it is still stronger than return. Fire blast covers steel before you get the TM for earthquake. Earthquake covers rock and steel, and the other moves cover other things. Note that the TM23 iron tail (and none of the other TMs in this answer) is obtainable only once in the whole game, so you can keep payback if you don't want to use the TM23 on this Tauros.
Brick break is an useful STAB move that has more PP than close combat and does not lower any stats. Shadow claw covers psychic and ghost. Swords dance raises physical attack, but I recommend using it only if you're hopelessly addicted to Voltorb Flip for some weird reason. Regular people should use aerial ace, which covers bug. Close combat is obviously a very powerful STAB move.
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