Excellent question.
"Bro" means something entirely different from "favorite," although they are similar. My favorite is Audino, but my bro is Breloom. Why?
"Favorite" is, believe it or not, more objective. "Bro" is based almost entirely on your own experiences with the pokémon. Specifically, if it's been reliable in battle for you. If Swanna kicked a thousand asses during your first BW playthrough, you might find that it's become a bro of sorts.
In contrast, "favorite" is more of an objective critique of a 'mon, and it considers aspects that you might overlook in your bro. For example, Audino is of my favorite type, has cool lore, and is a great subject for fanart. Breloom's typing is cool, too, but I don't think its pokédex entry is that special, and its design is good, but not nearly as much as Audino. However, I overlook these things, because I have many fond memories of Marty, my Breloom from Ruby, and I haven't actually used Audino in a playthrough yet.
So, to find your bro, you have to look back at your own memory. Think of all the games you've played and try to remember what your teams were in each. If there are any you can't remember, it's fine - they must not be your bros. Anyway, try and find one or more MVPs from each gen. The one that could take out an entire gym or elite four member on its own. The one that you brought in on the evil team leader, thinking all hope was lost, and held out hope for, even as its HP were reduced to red, until the last enemy fainted. The 'mon that was always a level higher than the rest, just because it always won its battles.
Got them?
Now, give it some time. Don't worry if your opinion changes - it's guaranteed to do so again in the future, anyway. And if you've got two or three bros, well, that's fine too.
It's even possible that your true bro is a 'mon that you haven't used yet - or even one that doesn't exist yet.
