I'm an old fellow...
Spent most of the 1980's playing the Ultima Series... IV being the first one i played, followed by III,II,I,V,VI,VII in that order... the less said about VIII and IX the better i suppose (IV and VII are my favorites of the series), they still run in DOSbox and i still play them from time to time. And i'd even
still recommend them!!
Plenty of agonizing fun with the old Gold Box games... but they're a mess to play without the materials the games themselves used to come with.
Also liked the Zork and Enchanter series (as well as Planetfall and Hitchhikers Guide to the Galaxy), the Wizardry series, and wasted all too much time
playing around with EA's Adventure Construction Set (Back when EA was an excellent software company... hard to remember that these days.) Along
with a HOST of old C64 RPG games, Bard's Tale and Phantasie being the few whose names i still remember (or still own)
A few computers later... Baldur's Gate I and II, Icewind Dale (The first, in particular i really liked)... and i played the lot of 'em to death over the years.
Planescape: Torment, and the Fallout games as well (I liked all of them up to and including New Vegas) all of 'em are worth playing.
I've spent the requisite time on the Elder Scrolls series, from Daggerfall on up, i'm sure they've been mentioned but they're worth a play as well.
If you have an old playstation console... The Suikoden series (if i remember right) used to be on the store there and it's one of my favorite old games
that never gets much attention. And one of the only games i really miss playing. One of these days i hope to see it on PC... Hope springs eternal.