Went with magnum zombie. I play a lot of UC and have had a lot of combos broken because of one badly timed shot.
Also I can kind of understand the reasoning behind why RPG J'avo would be dangerous. Magnum zombies though? Come on, they're brain dead corpses that should only be concerned with eating you, and somehow they put bullets into you with deadeye accuracy from half a map away.
RPG hardly hits me, because we can see the laser sight and have sufficient time to dodge. Those Magnum Zombies can hit us from anywhere, they knock us down to much, and, as we are standing up, they knock us down again. :X
Magnum zombies have unnatural aiming; they can hit you from far away and it's difficult to approach because of it and even put you into an annoying cycle of constantly being knocked down by them. Plus, they have body armour.
RPG J'avos are dangerous, but they are significantly stupider. The ones on Steel Beast literally land right in front on you. If you know where they spawn and when it's coming, it's a simple matter of swerving or sliding out of the rockets path when you're running towards it to kill it because it's not instant like magnum damage is. The only really annoying ones I've noticed are in Rail Yard. If you're in the wrong position, they'll likely destroy you with constant barrages of rockets.
Definitely the magnum zombies. They can hit you with pinpoint accuracy from across the map! The RPG J'avo are irritating, but they don't seem to ever move (or not nearly as much), thus once you figure out where they spawn, it's easy to take them down.