I would say :
1) ALL the Ultimas (since UIV) (They SO create worlds ...)
(Hole)
10) BG 1 & BG 2
11) Ultima Underworld 1 & 2
12) Stonekeep (only one, but VERY enjoyable - especially the fairy realm !)
13) LBA (Little Big Adventure) - Not a RPG really, but
14) Bard's Tale (Yes, I am an old timer ... So funny when suddenly 4 groups of 99 monsters were on the next 10'x10' area, not at all visible before ! But I HATED drawing the maps !)
15) Diablo I (first game I played multi with my hubbie) (Diablo II SUCKS !!!)
... And a lot of others ...