Lets weigh out the options here:
Big Show - Reported to be leaving, and isn't in top physical condition, so he's likely to lose.
Sabu - Has the least momentum of all after losing at Survivor Series and recently against RVD, however if he gets the table then he could unleash some destruction, but I don't think he'll be billed as ECW champion.
RVD - He's overcome plenty of challenges set by Paul Heyman, and is a fan favourite, but due to the drugs issue I think Vince will have him as a jobber in the Extreme Elimination Chamber but not flat out fire him, he's been known to use jobbing as a punishment before as he has done so with Triple H following the MSG Incident.
Bobby Lashley -Whilst he's made an impact in ECW since he came there, I don't think that he should win, but perhaps should be in the final 3.
Test - I think the only way Test should win is if he makes a face turn and screws Big Show if Big Show, Test and another combatent are in the final 3, he could turn on Big Show or allow Big Show to be defeated and then defeat the final combatent.
CM Punk - He's popular, a great wrestler and already has a rivalrly outside of the Extreme Elimination Chamber, if he wins he could develop his feud with Mike Knox culminating in a Royal Rumble title defense. He also has the respect of The Clique which could work in his advantage in terms of being booked as the champion.