OHaUH Gandalf vs Treebeard. Assuming an 8-round game where Gandalf/Treebeard enters play on the second round (the earliest likely round without the need of resource acceleration "tricks"). Also assume Treebeard is the only ent in your deck so you're not using his resources to pay for ents.
Here are the numbers of actions that Gandalf can take for each round:
0+2+2+2+2+2+2+2 = 14
Here are the numbers of actions that Treebeard can take for each round:
0+0+1+2+1+2+1+2 = 9
Comparison: Gandalf will get an additional 5 actions, but will cost 1 more resource and 12 threat. Also Gandalf only gets 2 actions if he is used for questing. If you have Treebeard in the deck solely for combat, then the actions come out to 7 for Gandalf vs 9 for Treebeard, but if you "like" the chump blocker strategy (Horn of Gondor, Prince Imrahil, Eomer, etc), then being able to use Gandalf to quest and saving another ally to chump block tends to end up being better.
At times - especially in solo - there may not be an enemy to deal with, in which case Gandalf will lose his 2nd action, but Treebeard will "store up" his 2nd action (though he'd lose his first if he wasn't questing).
The biggest difference that can be gauged without any bias or situational changes is the threat cost of Gandalf (and additional 1 resource cost). If the threat cost isn't likely to make the game any more difficult (your deck has lots of threat reduction), then I would say that it's a pretty close battle. But, due to this threat cost, Gandalf ends up being too costly to play in a lot of decks where Treebeard would excel.
So, I guess I'll tip my hat to Treebeard as the overall better ally. But if you have a deck that can support Gandalf well, there's no reason you can't have both.
I think the combination of these characters is quite formidable. They can practically handle everything on their own: Gandalf will quest, then Treebeard will defend, then they'll combine their attack to guarantee a kill. The heroes will be around to support these two characters rather than the other way around.