Which mods really spice up a game for somebody who has played many different characters and beaten the game many times over (and where can I find them)?

I'm looking for a challenge here.

Hopefully the challenge will continue in ToB.

unfinished business, tactics, ease of use, and ascension are must-haves

Try UFB, then tactics to see if you like it, then move on.

A challenge you say?

Tactics, Tactics, Tactics.

Quest Pack!

And some NPCs. Kelsey is good, though he won't add much in the way of challenge.

Especially if you get Valen. Be careful of some of the mod NPC's, I find them WAY overpowered.