Well, first, if you dont mind getting some engine upgrades, installing Baldur's Gate Trilogy would be good (requires BG2 & Throne of Bhaal, tho)
For BG1, there arent as many mods as for BG2, still some essentials would be:
NPC Mod for BG1 (adds alot of party dialogue & romances)
BG1 Unfinished Business (adds missing quests)
BG1 Quests
and for some of the bigger mods (they all add new areas & quests):
Dark Side of the Sword Coast
Northern Tales of the Sword Coast
Secret of Bonehill
Dark Horizons
Drizzt Saga
There are many more, such as individual NPC mods or gameplay/stats rebalance changes. Your best bet would be browsing the main mod sites and picking what you want, this link can provide you some mods for BG2 as well as links to other modding sites.
http://www.spellholdstudios.net/ie/moddingBesides our own site here, i would recomend Gibberlings 3 & Pocket Plane Group for BG1 mods.
Good luck!
