for modern warfare, i'll gladly pimp my TCW (The Cold War Deluxe) scenario.
it has everything and i mean everything. the full fury of land, sea and air plus 30 different civs to choose from. 2 eras of techs and about 300 total units.
of course, there are other scenarios out there that are excellent. Rocoteh's WW2 Global is awesome as is the Barbarosa and TGW scenarios. Plotinus' Rood&Dragon seems like a well-made scenario as well. if you like the turn-of-the-century/pre WW1 era then you might like my Age of Imperialism scenario.