I've only just recently gotten into the use of mods after spending several years with the unodded versions of all of the games. However these are my findings thus far:
The level 50 xp cap and Spell cap mods are quite handy, combined with playing through with a pc created party once or twice, and the use of some of the tactics mods on the second go around, these can make for some interesting game play. A+
Solafaein mod works nicely, he's got a lot of content, and interacts quite a bit with the other characters. Won't give much away, but can be given over to vapid moralizations too much of the time
... Good character, and good mage all around. A
ITEM mod, oh my god this is nice. This allows you to take about half that junk you let sit i nthe bags of holding cause you don't want to sell it (just in case you ever need it right?) and combine them and augment them using cespenar or cromwell. Some ToB level items can be forged from 2-4 SoA items, a few reagents, and a ton of gold. Also stuff like the hallowed redeemer which got kinda outclassed very quickly can be upgraded. I give this one an A+++
Valen mod, not used it much as I play mostly good parties, seems well developed however with some interesting special items and abilities. Also fills the need for a theif that isn't Jan and isn't that traitorous lout Yoshimo (I was so shocked my first playthrough... good times). Give it a A?
http://weidu.org/
These were put together by one of the dev's for Baldur's Gate, and as you might expect they are very polished, and well put together. Good things to have when you want to play a classic with a new twist.