Dave Gaider's Ascension (available at
http://www.weidu.org/ascension) is surely the Top Mod. Quite a bit harder than the standard ending, but much more interesting & giving a much greater sense of "completion", IMO. Since Mr. G. is credited as a Senior Designer of the published BGII, I reckon he has more right than most to meddle...
Personally, I'm growing increasingly suspicious of Mods - in particular, I think the majority of Tactics Mods are ridiculously difficult & give away too much treasure, so unbalancing the whole campaign. Maybe I'm just a weak player, though I do play very carefully...
My feeling (no offence intended!) is that a lot of Mods are developed by & for those who just want extra difficulty in Computer Game terms - they're only do-able by parties specially prepped for the particular challenge, which doesn't make sense if considering what the characters should know, etc.
That being said, I do use the Ease-of-Use, Item Upgrade & aPack (pro-Thieves) Mods. I like Shiney Ones - the more the merrier! And anything that cuts down the huge amount of time spent fiddling with equipment can't be all bad...
Perhaps a Mods section, with links & reviews, would be a worthy addition to this already-excelent Banshee site?