usual reasons:
- companions with personalities
-
a lot of optional content, including quite elaborate side quests
- well conceived antagonist(s)
- mage "duels"
- varied encounters and itemization
- class-specific content
- sense of an epic adventure, but not without "personal" edge
- well conceived big city
- well designed UI
- a lot of lore
Some, or maybe even all, of this could be argued not to be "that great" under scrutiny (like, personalities suck, side quests are mostly just combat (the most difficult quest in the game is Jaheira romance
), no C&C to speak of, lacking support for evil protagonists, the world feels more like an amusement park, etc.), but same could be done for stuff usually listed as BG1´s best assets too - for example, being able to explore all those maps whenever I want is cool and all, but due to slow walking speed doing so is tedious and there are just 1 or 2 interesting things to encounter per map, which means most of the time it´s just walking around while nothing´s happening, low level campaigning is cool and all, except archery is totally overpowered and low level mages are boring as fuck, etc.
Personally, I don´t really bother trying to decide for myself which of the vanilla games I prefer since I think both become much better when mods are allowed to be a part of the picture. For BG1 it would be mainly one of the mods "porting" it into BG2´s engine (either EasyTutu, or Baldur´s Gate Trilogy), BG1 NPC Project which makes all the running around a bit more eventful and Sword Coast Stratagems which diversifies the game´s encounters and ups the challenge, for BG2 it would be Sword Coast Stratagems II (shitload of improvements, including cool stuff like some high level enemies casting Wish), Rogue Rebalancing (enemy thieves utilizing detect illusions, etc.) and Refinements (more diverse and class-specific HLAs) + Ascension (substantially ramps up difficulty of ToB´s major encounters and thus makes all the leveling and powerful loot more "meaningful") for ToB. Also 1 Pixel Productions´ avatar and paperdoll changes, because these indeed look a lot worse in BG2 than in BG1, and Widescreen mod for obvious reasons.