Opinions will differ, but generally people really like the first and/or third. The second was rushed, and it shows, mostly by a ton of reused assets and locations.

The first is more loved by hardcore or oldschool RPG fans. The third is more action oriented, generally a lot more colourful, and very much larger. Its combat is generally liked more than the first (except, again, by CRPG fans).

All three have memorable characters.

I'll be a contrarian and throw in my vote for the second game - it's rushed and flawed and the asset reuse is blatant to the point of being legendary, but the setting and story are the best and most original of these 3 games. Just being a hero of one single city instead of the entire world is surprisingly refreshing.

In general I'd say that 1 has the best combat/tone, 2 has the best setting/story, 3 has the best characters. I've heard that 3 can be quite enjoyable if you pretty much only do the main story and companion quests - but I wouldn't know, I'm one of the poor fools who got stuck in the Hinterlands, and that mistake + the very underwhelming main story sapped my will to continue playing.

Honestly, they're all at least decent. Which one is the best will vary drastically from person to person. Personally, I liked 1 the most. 2 was decent. 3 was.. playable, but with some extreme flaws. 3 is the only one I never finished.