Well the absolute IDEAL melee class to do it is a Frost Deathknight, but we didn't have one of those and Contagion hates frost anyway
Essentially you have options. You can get a frost mage to do it, but that will probably cause enormous DPS issues, most likely in phase 2. Hunters can do it as well - I hear that works pretty well as they can throw some damage on the boss too.
Essentially though it is a plate melee setup that we used. Ret pallies have Holy wrath for an AoE stun, they have turn evil which allows them to fear kite, they run at 15% speed and have Judgement sprint too if they need a quick boost, they have good personal healing if necessary and some decent defensive cooldowns. It's hard because you can't really tank them as a plate dps - you'd probably just die, so you need to kite them. The plate is just there is it gives you that extra cushion.
You don't HAVE to kite them in P1. You can tank them, indeed that is really what makes it much harder on 10 man than 25 - 25 man allows you to have a full tank on the adds, whereas 10 man makes that more difficult.
One thing we tried was having Klog tank the adds in P1, with Uru tanking Nefarian and then Contagion tanking Onyxia in DPS spec and gear but in Deathknight Blood presence. Feral cat druids can do this too - hop into bear form and tank her. Ret Paladins would probably just take far too much damage, I expect fury warriors would as well. This makes Phase 3 abit easier iirc, as the adds will all die in one spot in P1 and you can control it exactly. However it also makes phase 1 healing utterly horrendous, with a non tank spec DPS tanking onyxia taking shed loads of damage, the add tank taking shed loads of damage, the tank taking damage from nefarian and the raid taking massive damage from Crackles as well.
But mainly Darksen is awesome