Some great mentions in the lists here, I'll have a go at my top 5 based on nothing in particular, in no particular order, and I'll exclude XCOM games.
Sensible World of Soccer - Such a beautifully simple game. I have probably played this game the most out of all games.
Frontier: Elite 2 - Blown away by the scale and scope of the game, at the time. In retrospect shuttling back and forth from Sol to Barnards Star probably wasn't great gameplay, but it captured my imagination massively at the time.
Jagged Alliance 2 - Picked this one up only in the last couple of years, but WOW. The game designers got so much in the way of playability, humour and sheer character in the game world. Just a beautiful, fun game, and only made better by 1.13.
Tie Fighter - For a couple of months, about 10 years or so ago (longer?), me and Lord Vader were tight. I swear, I was there.
Dwarf Fortress - For the ambition, creativity and madness of Tarn Adams. Under normal rules of game development, this game just would never exist. Have sunk plenty of time in to this one, probably a distant second from SWOS.