ThermioN: ... and wing commander (i'd love to see this on gog...)

Aliasalpha: I have to join in with the IL2 Sturmovik crowd. Apart from being one of the most detailed combat flight sims ever made, it has some of the best potential for insane fun.
Flying a HE111 inverted under a bridge at an altitude so low its tail becomes a rudder
Putting on unlimited ammunition and spamming ME262 rocket packs at incoming multiplayer opponents (agreed in advance naturally, no cheating involved) or using one of the IL2's bomblet packs and just doing barrel rolls while constantly dropping them, like a giant cloud of death (or maybe like a low tech version of that Death Blosssom thing from The Last Starfighter)
Firing a few cannon rounds into a bomber carrying a 5000lb bomb and detonating it, fragmenting not only it but the squadron and everything else within half a kilometre (often including myself)
Not so relevant now that the game isn't being patched but it was always amusing to load up the saved .track filess just after a new patch came out. They invariably changed the flight model of various planes and since the .track only record control input, the new flight model often meant a radically different outcome than originally recorded. If you wanted to make sure you saw the replays as intended, you had to remember to save as a .ntrack before you patched
Picking the most insane multiplayer matchups and seeing just how well they work together. You'd be surprised how long an I-153 can last against an ME163, both targets are so damned hard to hit because of the sheer difference in speed. Hard turning dogfights between bombers is an awesome game, trying to have a knife fight between planes so big and slow can be remarkably challenging, at least until one participannt gets above the other and starts dropping dozens of tiny bombs...
There's also the fun of actually playing the game SERIOUSLY!