When it comes to power-armor, its rock-paper-scissors with over-sized hydraulic fists. There are a couple of extraneous models sitting on the sidelines, but the big ones one, two, three - shoot!
An Eisenhower is what most people think of, its the base-line, middle, rock. Unlike a vehicle, when bullets hit you, you will get hurt. Unlike normal body armor, it does a decent job of absorbing the bullets. Think something that would give you additional hit-points as well as damage reduction if you were playing a game on the SPHERE. The suit won't make you superman by any means. Don't try to fire a Baby-Three (It replaced the "Ma Duce" -what did you think we'd call it?) from a standing position - you will end up on your ass. But the point is that you can that lug a fifty-cal machine-gun for a few dozen kilometers rather than a few dozen meters before your arms fall out of their sockets. You also get improved sensors and decent air-conditioning in the bargain.
The Ike is like a space suit for a big man, the MacArthur like a diving suit for a gorilla - no mistaking them. Mighty Macs are built to counter lesser armors. Thicker carapace, stronger hydraulics, multiple anti-tank missiles, light autocannons that you can both carry and fire. Its more of a one-man IFV or light tank than an armor, though most of the same rules apply. You will feel the hits you take, but less than if the armor was lying right against your skin.
Now you can't shoot what you can't see, and that is where the Donavan fits in. This does look like something fit for a superhero - complete with cape - and it does have chameleon and stealth systems that are beyond normal human capability as well. However, the suit is there to support the systems, and you're there to support the suit - you really don't get the strength and endurance boost of the others, nor most of the extra heavy weapons. A few boom tubes or rocket launchers will still put a dent in someone's armor, and rest assured, you can get close enough to use them, but it really won't take the hits if they shoot back.
Waiting on the sidelines are things like FARPS - Forward Area Resupply Personnel Systems, if I recall correctly. Think the super-hydraulics of the Mac with even less armor than a Donavan - less ubermencsh, more uber-forklift. There are also power augmented diving suits, and variants designed for HALO parachute insertions. You're not too likely to see those even the big three - they're pretty maintenance intensive when you get down to it. Maybe someday we'll be abel to have a fully mechanized infantry unit, but it will always be exactly one day after you get skragged.