The fight I want to see is the IoM vs the Zentradi.
Both sides are made for only war, but its finally a matchup where the Imperium doesn't have a numerical advantage. The IoM even has a significant technological advantage over the Zentradi in a number of aspects (Zentradi don't fix their ships and don't have shields) but the Zentradi have the advantage of ground warfare being inconsequential to them (only a handful of Imperial ground weapons are of any threat against the Zentradi, space marines are no threat to them at all) and travelling around in fleet sizes that would make the Imperium shit itself, and they need a reason NOT to glass enemy worlds they fight, since they are entirely spaceborn and couldn't give less of a shit about controlling planets.
The Zentradi showed up to Space War 1's final battle with more than 4 million capital ships alone, not even counting escort ships or attack craft, and those ships are of comparable size to the larger IoM vessels. This is a single Zentradi armada, one of an estimated 350 different competing groups scattered across the galaxy.