Final Fantasy VI is an easy one: the Game Boy Advance version. Good retranslation, classic graphics, and an extra dungeon in the post-game if you really liked it. The sound has some quality issues that bothered people enough to make a patch for the rom, but it never personally bothered me.
Other versions of FFVI are the SNES (fine, but no extras); PS1 (no extras except a couple of FMVs that mean nothing to you; also horrible loading times), and PC/mobile (terrible redesigned graphics and menus designed for a touch screen). Just play the GBA version.
Final Fantasy IX, on the other hand, is a slow game through and through. It's very good, but I'd really recommend you start with 6. Anyway, I've heard the Switch and Xbox One versions have weird slowdown here and there, so that's out. The Android version seemed fine to me. If the PC version doesn't have slowdown problems, I'd just suggest that one. The original PS1 version ran fine, but I don't recommend it just because it doesn't have a speedup button like in the modern releases, which you'll really need, since I'm not joking about FF9 being slow as fuck.