Gaderael said:
FPS: Serious Sam, definitely, though maybe the flood in Halo:CE.
RTS: Medieval: Total War, hands down.
Another vote for serious sam.
Exhibit A:
http://i73.photobucket.com/albums/i212/Abu_Dhabi/SS2a.jpg
I put just a link on purpose, as you need to see the screenshot in a full window to really get its effect. Bear in mind that, unlike a lot of game featuring mobs of enemies, every hit from these guys
hurts.
Edit: In retrospect serious sam doesn't have the highest raw enemy count, but to my knowledge, it has the highest enemy count of deadly/strong/fearsome enemies instead of the usual fodder most games have (I.E. Kingdom Hearts II)