My vote goes to Tactics mod.
A couple of battles with Improved Irenicus and Ascension on Insane were just (...looking for words) incredible, breathtaking, close to nervous breakdown, sweaty, kick ass enjoyable!
Ridiculously hard is only for the time until you learn the TACTICS!
I used to just click on enemies with vanilla SOA on Core. Now with Tactics on Insane I actually learned what most spells do and how to use them. And it's not so hard anymore. Tactics has a lot to do with defense and offense comes later. I assume that's the problem for most players. They just want to click on the enemy again and hope to win. Unless you protect yourself you are soon history. And yes, that is only possible for certain classes. So if you want to roleplay, you can: set difficulty to easy, uninstall Tactics or have a lot of reloads.
The way I see Tactics is - who is smarter: mr. Weimer or me? And it challenges me to win difficult fights. Many of them can be won with cheese (Spike Traps, Mislead backstabbing, Project Image abuse, ...) which of course many of us do just to win for the first time or two. But later (at least for me) I accept the challenge and play the game the way Weimer meant it. And even later I added the biggest roleplaying factor - no reloads. Now it gets really interesting. I enjoy these games immensely.