My favourite games are
Sacrifice, Fallout and
Age of Decadence but if I had to go with one I would go with Sacrifice because:
a) It's greatly underappreciated
b) It aged very well (none at all to be exact, it looks beautiful even for contemporary standards)
c) It's utterly unique, something out of this world
d) In no other game have I felt this powerful (not even in games where I was literally a god, e.g. in Black and White)
e) It has fascinating story, characters and witty dialgoues
f) I can play it all the time without getting bored and I get back to it every year
I could praise it for hours but the point is: if you hadn't played it you don't know what you're missing.
As for my least favourite games I would say all Beamdog mods. I've elaborated on this here:
https://www.gog.com/forum/planescape_torment_enhanced_edition/beamdog_is_the_parasite_of_the_industry