For bf2: Point of Existence 2 and Forgotten Hope 2 are a mustplay.
For hl2: Minerva , all episodes. There are alot of crappy singlemap mods for this out, alot of the full conversions have few players actually playing or are plagued by lag/bugs. I played sourceforts for a while but I just can't stand the retail hl2 weapons anymore. If somebody knows proper well played hl2 mp mods with actual fresh weapons please let me know.
For bf:vietnam: Point of Existence 1 obviously.
For bf1942: Desert Combat, had alot of fun with it. I never really got into fh1 much since only the hardcore players played it when I found out about it. The ones that scream noob at every question and teamkill you with a satchel for entering "their tiger", you know the kind.
For Stalker: I used a mixup of alot of different mods so can't really point out what I used. I suggest using the weapontweak minimod files, combined with some minor mods that enable repair at shops, a realistic wear on armor mod and some mods that fix the constant bandit spawns ( these were a pain and a terrible game element imo )
Diablo II+lod: minimod giving a little extra room inventory/chestsize wise, plays alot more fun imo.
HL1: brainbread. Great zombie mod, still servers playing this so if you're bored I suggest you try it.
COD: united offensive: there are some servers playing a mod with more kinds of tanks, new maps, halftracks, AT-guns, etc. Only played this mod a few times but the experience was terrific, don't know the name though.