I've only had three partners in doubles since Brawl came out: Snake, DK and Lucario.
DK was by far my favorite, he's a ****ing tank. His ability to take damage, combined with his heavy hits and the fact that he can gimp early in a match (spike, cargo toss stage spike), made him the best doubles partner I've had. The worst we've ever placed was second, and that's not bad at all.
However, Lucario isn't that bad either. The only Lucario player I've teamed with was really smart--he rarely used his up-b to return to the ledge, spammed AS all day, knew how to live FOREVER at high percents and saved his ridiculous f-smash as a kill move.
The upside to having Lucario as a teammate is that his dodge roll is practically instant, the more damage he takes, the more damage he gives and you BOTH have a counter.
The only downsides I can think of can be avoided:
-His recovery shouldn't be gimped in a doubles match if his partner is aware of what's going on with both of them.
-Lucario gets a lot of his knockback at higher percents--smash DI, camping and that stupid dodge roll can all aid Lucario in living longer at high percentages--they just need to be smart.