It depends. Brotherhood's campaign is one of the weakest, at least to me. But, the game is redeemed by its really cool machine missions. That said, Revelations ditched a lot of ideas introduced in Brotherhood to do its own thing with bomb crafting, hookblade and... yeah. At least, Constantinople is one of the better cities we've had since Venice.
AC3 may be the best AC I've experienced, as a whole package, but the campaign drags on forever and never achieves anything worth getting through. Fortunately, the combat is interesting, so is the naval aspect.
Truth is, I think that any of the AC games will provide entertainment with their own merit. AC2 was really the best, so after that, it's really up to you.
Brotherhood is the best AC game to date, so don't skip that one. Revelations is entirely skippable and -despite what the name suggests- nothing important happens at all. Maybe play AC3 because it's at least a little bit different to what has come before, but it isn't in the same league as Brotherhood.
Just play Brotherhood and then leave that series alone (unless Black Flag is able to redeem it). After playing Brotherhood I was so in love with the AC series that I thought I'd play Revelations in preparation for AC 3, I knew that it wasn't as good, but I thought my desire for more AC would still make it enjoyable. Sure I had some fun with it, but it paled in comparison to AC 2 & Brotherhood. It also had the added effect of making me not want to play AC 3, which I don't actually think was a bad thing. Really, Revelations was just incredibly disappointing compared to what came before it.
Play Brotherhood its great. It basically take everything that was good about ACII and makes it better. The story is a little weak though since its mainly about Desmond's story and not Ezio and Desmond is honestly just boring. I think you can skip Revelations unless you care about Desmond's story because it will explain everything that happened at the end of Brotherhood. Haven't played ACIII, but the consensus seems to be that its disappointing so i guess skip if you don't care about the story.