IMO, it's between the Executioner Axe and the Fullblade... And sometimes a Mordenkrad.
Fullblade: The extra +1 to-hit is kinda wasted, IMO, but it fits the stats better for a Pursuing Avenger, particularly if you keep Strength boosted pretty well, too.
Executioner's Axe: My fave. You don't need the extra +1 to-hit, and anything that does more damage is pretty cool for an Avenger. OTOH, you need Con + Str for a lot of feats, and you probably won't have either.
Although the Mordenkrad is my very favorite Exotic 2-H weapon, it usually sucks for Avengers - with one caveat. Sure, you get a meager damage boost, but without the High Crit property, you're missing some damage potential. Also, it's still Str/Con for feats, and Hammer Rhythym does you no good at all, anyway. With that said, a Crusader's Mordenkrad is potentially awesome if you have some Implement powers and a budget.
-O