For a generic terminal to use with pretty much anything, it's hard to go
wrong with an HP 700/96 or 700/98. They have great DEC emulation, full
HP blockmode/forms support (required if you ever want to connect it to
an ancient HP system running a non-UNIX operating system), and unlike
DEC terminals—which are all seen as "desirable"—the "modern" HP
terminals are on the inexpensive side.
The downside is that, unlike the very last models of DEC terminal
(VT510/VT520/VT525), they still use a custom keyboard, but there are
both PC-101 and HP-ITF layouts and the keyboards aren't that "desirable"
so, again, they tend to be inexpensive. And personally I like the HP-ITF
layout if I ever need to use vi since the escape key is in a pretty
The more important thing is to set up a decent RS-232 port on the
Raspberry Pi (or whatever else you use it with). Three-wire serial
(transmit/receive/ground) will work just fine with XON/XOFF in-band flow
control as long as you adjust the levels between TTL and RS-232, such as
with an MC1488/MC1489 pair or something like that, but to really fly
you'll want full hardware flow control at proper RS-232 voltage levels.
(The latter is something I've heard some USB-serial adapters fail at.)
This guide has a lot of good and detailed information: