HECU Marines: http://half-life.wikia.com/wiki/Hazardous_Environment_Combat_Unit
Situation #1: The Hazardous Environment Combat Unit has regrouped with Cpl. Adrian Shephard(http://half-life.wikia.com/wiki/Adrian_Shephard) as their leader. They have all the tech available to them from half life 1. Vehicles, weapons, armor, etc. They have rebelled against the Combine. Can they succeed? Also, this is before Freeman or anything that happened in Half Life 2. Aside from the Combine takeover.
Situation #2: Same as situation 1 except that the Black Ops(http://half-life.wikia.com/wiki/Black_Ops) have allied themselves with the HECU Marines. The Black-Ops also have all their tech available to them.
Now for my personal opinion. I loooved the HECU Marines in Half Life. I think that they are some of the best and most entertaining enemies ever seen in a video game. That said, I've never been able to take Combine infantry very seriously. However, with all the tech available to both sides, this will be a close match. It's up to you guys to convince me of the winner.
Edit: Remember, this is a war scenario. Not a lone battle scenario. There is a difference.
Edit: Hey guys, I need some comments. I can't decide this thing on my own.