Exodus is pretty good on NES, but don't play anything else on NES. The best versions of Ultima IV are the Amiga and Atari ST versions (which are pretty identical). Failing those, the SMS version is a much better port than NES. Not sure about any remake.

Ultima V is best played on Atari ST, but not Amiga. The second-best version is the Commodore 64/128 version, in C128 mode (on a C64 there's no music). Do not play the NES version under any circumstances.

Ultima VI is best on PC, although the Amiga version isn't far off. The SNES version is actually decent as well.

After playing Ultima VI, I recommend taking a little detour and playing the two Worlds of Ultima games (PC only). They barely qualify as Ultima games, but they're very good, and this is the appropriate interval to play them, because they use the Ultima VI engine.

Ultima VII is amazing, try to get the full version with both games and both expansions. Don't even think about trying the SNES version.

After that, the two Ultima Underworld games are fantastic, albeit different. They're first-person RPGs along the lines of Dungeon Master. I don't recommend the Runes of Virtue games.