Avernum: Escape From the Pit.
I am definitely a new player here compared to almost all others. I find it kind of amazing because I was hip deep in RPG's back when the first Exile came out, and yet somehow I entirely missed these. It wasn't until Escape From the Pit was on a STEAM sale in 2012 that I even found it. Friends that never played anything from Spiderweb Software, but still thought they knew something about it, told me that I needed to play at least one of these games to have any 'Indy RPG cred'.
Either way, I loved it. Finished all three Avadon's and Crystal Souls too. I have a backlog to clear out but Ruined World is in there soon enough, and at this point, I'll play anything that Jeff Vogel makes.