Operation Flashpoint - I played it when it came out and I guess I never will stop playing it thanks to great gameplay mechanics, fantastic mission making and modding potential and tons of user made content.
Doom - I'm a big fan. I like all versions in their own right and can easily return to it.
Street Fighter II - also a big fan. My personal favourite is probably II' Turbo: Hyper Fighting Japanese version for both, Arcade and SNES. I love playing it against good players even if I'm losing.
Disciples II - I don't play much anymore but was pretty addicted to it a few years ago. Good mission making potential too.
Mortal Kombat II - pretty addictive but I lost a bit of interest because the fighting system isn't all that great.
Super Punch-Out!! - very addictive. Can always retrun to it too.
Super Hang-On - very hard to master but that's it's charm. Frustrating addictive fun.
F-Zero SNES - while F-Z X is good in it's own right (I guess especially with the 2500 mph hacks and track editor from 64DD expansion), I personally prefer the SNES one. The lack of multi player is probably it's biggest problem but there exists a PC program to play it with multiple players. (F-Zero VS)
Super Mario Kart - pretty addictive in MP and time trails. I don't play the newer ones.
Test Drive II - ciassic racer that I enjoy. I prefer the SNES version to other ones.
The Need For Speed - the DOS version is my favourite. I also like Porsche Unleashed.
various shoot em ups - I don't like the "bullet hell" ones too much, I prefer those in the vein of Darius Twin (SNES), Raiden Trad (SNES, MD, Arcade), 1942 & 43 (NES, Arcade), Axelay (SNES), M.U.S.H.A. (MD), Gaiares (MD), Thunder Force IV (MD), ect.
various NES/Famicom games - Super Mario Bros. 3 (the only Mario I can always return to), Contra & Super C (best Contra games for me and imo better than the arcade ones), Rush n Attack/Green Beret (I like NES version best), Punch-Out!!, Lifeforce/Salamander, Jackal, Gun.Smoke, Guerilla War (better than arcade imo), Baloon Fight, Battle City, Antarctic Adventure, ect.
various simulation games - I like especially the old polygon 3D ones but I also like the more modern ones.
EDIT: I also enjoy various tactical shooters and arcade style shooters a lot.