Putting in a plug for a Uxie set I came up with that has been a great momentum grabber.
YUP... Uxie.
Uxie @ leftovers
Ability: Levitate
Nature: Careful (+SpD, -SpA)
EVs: 248 HP, 192 Def, 68 SpD
-
Yawn
-
U-Turn
-
Protect
-Stealth Rock / Light Screen / Reflect / Psychic*
*-if you use Psychic, change nature to Sassy (+SpD, -Spe)
Uxie has a very unique movepool that allows it to play the role of a defensive momentum grabber. It will survive just about anything once or twice (typically more if you play it right). The idea is you bring Uxie in on something that will do pitiful damage to it (a surprisingly lot of things), Yawn, then U-turn. If they stay in, most things are faster than Uxie meaning it tanks the hit and your next pokemon comes in on a sleeping foe. If they switch, just like any other momentum grabber, you pick your best counter. Uxie is particularly effective against known u-turners and volt switchers... after the yawn, the opponent will 99% of the time use u-turn/volt switch, which Uxie protects against.
The main advantage Uxie has over offensive momentum grabbers is that Uxie is much much safer. Offensive momentum grabbers risk taking heavy hits or outspeeding the opponent leaving the possibility of your switch-in getting hit hard. Uxie does a great job scouting attacks with protect and giving your team the perfect opportunity to take control, mitigating the total damage your team takes, grabbing the most possible momentum.
The lack of reliable recovery is definitely a hindrance which can be somewhat mitigated by protect, but with 353 HP and 344 in both defensive stats, Uxie can really take a pounding, plus add its ablility levitate, giving it the ability to switch in on those ever so common earthquakes, and you are left with a very survivable pokemon that can grab momentum several times over the course of a match.
Beware of Status absorbers and choose your opportunities wisely. Luckily, if your opponent catches on to your Yawn games, you can U-turn right away if you smell an incoming status absorber, still gaining momentum. This build is excellent for mind-games.
Like most momentum grabbers, it pairs well with stat-up sweepers. A wish passer is always welcome to keep Uxie in business longer; but its not typically needed, as Uxie is support and can eventually be sacced to make way for you late game sweepers. That being said, YUP... Uxie is a team player, and a great choice to build a team around.