BG1 for me.
While I enjoy BG2 very much, I feel it goes overboard with the amount of quests you get handed to you and the general powerlevel of the items/monsters/players. You hardly step foot outside the first area and vupti you have many quests filling your purse and already your first +3 and +4 weapons....... +4 possible
at Level 7-9?
Had it stayed a bit more closer to (A)D&D pace, I would have loved BG2 above all else as a game, but now it ranks just below BG1 for me.
I like BG1s exploration, atmosphere and pace much more (when you do find thoese rare magical items it is fun), the tactical aspect of being able to be killed by a few stray arrows so you need to move slowly, scout and look for traps.
BG2 however had better party interaction and .... well, more of everything which as mentioned above was not all good.
So I prefere BG1 over BG2, but both games are leauges ahead of most every other CRPG I've played.