Avernum 6! Exactly like all the other Spidersoft games - huge, entertaining and quite challenging at times. I have to admit I haven't finished a single one of them but played them all extensively.
You can get a "complete saga" package on gog.com for 11$, includes avertnum 1-6 and the Blades of Avernum. This has to be a better investment, assuming you want to invest your time into a classic rpg game, than baldurs gate complete, when you consider both the price and the amount of gameplay time you're getting. These games really are big. And realy are fun.