Well, no matter how many times I tried, I could not get into Mass Effect. I loved Knights of the Old Republic, and I really wanted to like ME, but there was just something keeping me from enjoying the games....
Bethesda--Morrowind is one of my favorite games. Oblivion was really good, but I did not enjoy it as much. It was bigger...but felt emptier. Fallout 3...even more so
Square--Chrono Trigger is one of my favorite games, along with Final Fantasy 9, and Kingdom Hearts. But...I haven't really enjoyed the double-digit final fantasies, and I felt that the Kingdom Hearts sequels brought down the series (complicating the plot while watering down the disney worlds...not a good sign).
Obsidian--KOTOR2 is the onyl game of theirs that I've played...it was fantastic (imo, better than the first if you keep in mind all the meddling of LucasArts)
Can't comment on Ion. I bought Deus Ex when it went on sale, but I haven't gotten around to playing it.
While these are all games I've loved, I'm gonna half to go with the developer that's consistently made RPGs I've loved (even when one of their series started lacking for a bit, they had more to pick up the slack). From pokemon to paper mario, Nintendo's RPGs are usually fantastic. Pokemon for its huge amount of hidden gameplay depth (knocking TES's "create a class" and FF7's materia system out of the water), and the Mario RPG titles are have hillarious writting and usually contain an interesting twist on the turn-based gameplay.