Sykar
Arcane
Throne of Bhaal is as challenging as you want it to be, at least with SCS2 installed. It's a cryptic answer, but the most credited one.
No encounter, save a few enhanced endboss-tier scrums (Improved Demogorgon, Ascension Final Battle) can stand against the might of a fully-rested party piloted by a competent player. They won't even stand a chance in many cases, because so much firepower can be brought to bear on them. And this is without indulging in serious cheese, only using "fairer" methods of offense/defense (no Project Image/Simulacrum cheesing to have effectively infinite uses of consumables or whatnot). And, in a stroke of brilliance, Bioware decided to remove any disincentive to rest up after ever encounter; the Pocket Plane makes this a perfectly safe option with no downsides spawned by RNG. Hell, the game even advises the player of this in the combat feedback log when ambushed.
That said, if one restricts resting, SCS2 Throne of Bhaal can be quite fun. Try going through each Bhaalspawn's base without refreshing spells. Instead of being able to bring all guns to bear against every encounter, the player is forced to ration limited-use abilities, conserve spells, and make heavier use of consumables. Going through the ~7 maps of Sendai's Enclave can be a bit draining, and Abazigal's Lair will pit the player against (up to) nine dragons* in one rest (sometimes in pairs). in addition to other enemies in his cave network. This forces one to squeeze the most value out of everything, because saving the "big guns" for the tough fights ahead becomes an overriding prerogative. It's definitely a little metagamey, but if you can stomach it, a lot of fun can be had.
*Probably my favorite addition in an SCS2+Ascension install, especially theFour dragon fight, which is really fun. Ready the Spooks, Symbol(s): Fear, and Insect Plagues!with two blacks, then a green, and then a red...with no time in between!
This is how I played the entire BG series.