back from a long think over this and i have concluded the flood and forerunners would be the only ones that stand any kind of chance as a lot of people are thinking the main couple from the star trek federation Klingon ect ect but when you put the federation, Klingon's, romulans, Borg, Dominion, cardasians, breen, species 8472 as just the start halo is out numbered by stupid numbers as each race from star trek has a fleet the size of the covy's so in space it's a ratio of 3:8 (flood doesn't have fleet) even with spartens it would be a masacre because it would only take one borg to steal the forerunner tech and then it would be over with only the flood left that could cause problems (althought not sure what would happen if 8472 got flooded up)