I used to play the old Avalon Hill boardgames to death, especially the big classics like Third Reich (I have all three versions, the newest is great), and Squad Leader (only the basic version, not the monstrosity ASL has become).
I see Magic Realm was already discussed, another classic.
Republic of Rome! Now there is an old game that doesn't get a lot of press, from the later days of AH. A very involved game, despite its simple appearance, and well worth the enthusiasm displayed here. You really need a merry band of cut-throats to play.
Maybe I missed it, but I'm surprised not to see Civilization (I have the original and all the expansions) and most of all Kingmaker not mentioned. Kingmaker is perfect for abstracted campaigns, and can be esily house-ruled by adjusting the force cards, events, etc.
There are too many old classics to name, but Starship Troopers was another that still has great replay value. The "block games" from Columbia Games (the Harn guys) are also well worth the playing time.
Did anyone play the "Standard Games" games like Cry Havoc or Samurai Blades? Here's a good web page for them - these games would grab any skirmish gamer or rpg'er:
PS - oh! anyone play the Imperium Romanum II? Played the hell out of that one - strategic level play with scenarios covering all of Roman History.