Well the reason I'm saying UNSC is:
1. Master Chief.
2. Kinetc barriers will only work against projectiles, not energy weapons.
Most covenant weaponary and the Spartan laser will negate them.
3. The AI on UNSC ships is much superior to the VI on Alliance ships.
4. A UNSC cruiser is the same size as a dreadnought, with their frigates being around the same size as a cruiser. Plus UNSC ships can enter an atmosphere freely.
5. How well could an alliance soldier really stand up to a scorpians main cannon?
1. Not a valid argument. As soon as you bust out the hero of a story the other side is going to do that. Like nuclear weapons. You could have a space chimpanzee on your side and if he was designated hero he'll win against anything. So not a valid argument.
2. Almost all of the weapons in Halo have shown they behave as projectiles. Lasers would be the exception in this case, they would likely be the only weapon of any use. Even the plasma rounds fire and act like fancy bullets.
3. This isn't valid. We have no idea of the extent of a VI programming. Cortana is an exception to the rule. She is filed under a hero and will not be used. Again I could just bust out Edi.
4. Size doesn't matter when your main gun can rip through anything they have. It's like saying look how big that castle is, and then one tank shot rips the walls to hell. Size doesn't = power. They do have the advantage of entering the atmosphere, but any Alliance frigates/cruisers capable of following them in have guns capable of destroying them.
5. I would imagine just fine. Or possibly not at all. Shields are designed to protect all of their force stopping a few tiny precise projectiles. Something as wide and explosive as a tank round might simply go around any areas protected. If they were expecting tanks they would bring in a Mako. That thing has been shown capable of damaging frigates. The gun on that thing is probably insanely powerful compared to any tank the Halo universe could field. I know from a few references there are typical tank designs in the ME universe, but we never see them. My bet is on the Alliance on this one.