BG1 EasyTutu. Don't worry, it's a lot easier to install than it looks at first glance. Basically what the Tutu mod does is take everything from the first BG game and port it into BG2's superior game engine. This means better visuals, better gameplay, more class and race options, plus even more importantly access to a whole library of mods designed specifically to enhance a tutu-fied BG1.
There's another mod out there called
Baldur's Gate Trilogy which also ports BG1 into BG2's engine but which also tries to combine the two games into one experience, whereas tutu keeps the two games separate. Personally I prefer tutu myself and would definitely recommend it for a first playthrough since it tends to keep things more original than the trilogy mod, which tweaks a few game elements to make them all work together. But it's a personal preference and both Tutu and Trilogy will work with the following recommendations;
BG1 NPC Project. I honestly can't play the original without this mod anymore. It's why I haven't bought the Enhanced Edition yet, because they've yet to have made this mod compatible and I really don't feel like replaying BG for the billionth time without it. What the NPC project does is simple: It makes everyone in the game much more chatty. Tons of new dialogue, new character interactions, new decisions to be made and relationships to be forged. It's all done extremely well and is almost entirely seamless to be point it's difficult to tell what's original dialogue and what's been added save for the obvious telltale sign that none of the added content is voiced, but then the original game rarely had voice acting anyway so it never stands out.
Baldur's Gate for all its charm doesn't really have the same quantity or quality of character interaction like BG2 did, in truth playing BG after having played BG2 can be tricky because the original game just seems to much more... quiet compared to the sequel and more modern rpg. That's where the NPC project comes in, and it's so well done I recommend it even for a first playthrough. It really does improve the experience while always staying in the original games spirit. I can't recommend it enough!
Another important mod to get is the
Unfinished Business mod. It just adds in a lot of cut content, fixes a few quests, and generally improves the BG1 experience. There's absolutely no reason not to download and enjoy.
Again, these mods require one of the two above
(Tutu or Trilogy) to have been installed and
will not work with an unmodified version of Baldur's Gate.
One last mod to point out is the
Widescreen Mod which, obviously enough, gives the games widescreen support. If you're not using a widescreen monitor it's not worth downloading as BG2 supports pretty high resolutions even without and once BG1 is ported to BG2's engine, so will it, but if you are using widescreen then it's definitely worth downloading for a better visual experience.