I like this team. The Pokemon have good synergy between each other. However, you do have a major stealth rock weakness and have no spinner. All my changes will be bolded.
He seems like a very effective tank. However, there are some things I would change:
Mudmonster (Swampert) @ Leftovers
EVs: 196 HP / 224 Atk / 88 SDef
A different EV spread. He already has enough HP to survive a while. Slight extra investment in Atk provides more reliability in taking down Grass-types with Avalanche. Extra investment in SDef provides extra stability against Giga Drain and Grass Knot, normal 1/2HKOs.
A very effective physical sweeper. I love this guy. He provides lots of extra coverage on your team and is a good revenge killer (in a way... no choice scarf but still). No changes.
Nice wall. I personally have never used Registeel before in an UU team, but I will try my best... so I think this guy needs some work.
SpiderFace (Registeel) @ Leftovers
Trait: Clear Body
EVs: 252 HP / 100 Def / 156 SDef
- Thunder Wave
- Stealth Rock
- Seismic Toss
OK. Get rid of the IVs. Next, a different EV spread. Keep the 252 HP EVs. Because Cofagrigus is a huge physical wall, you need not invest as much in Def. Therefore, the rest may be added to SDef, as Cofagrigus is not fully targeted towards being Specially Defensive. Next, go with Seismic Toss instead of Earthquake. Because of Registeel's low base 75 Attack, he won't be dealing huge chunks of damage. Seismic Toss is great to take down physical walls or Pokemon that will try to set up in front of it.
One of my all-time favorite walls and cripplers. Cofagrigus is a great check to all physical walls EVERYWHERE, and he has great synergy with Registeel. I do have some suggestions, however.
Tut-tut-tut (Cofagrigus) @ Leftovers
EVs: 248 HP / 204 Def / 56 SDef
- Pain Split
- Shadow Ball
Alright. I'm changing your whole strategy with this guy. His job is a Defensive Wall and crippler with some investment into Special Defense. Leftovers as a new item provides slight recovery every turn. A new EV spread turns it into more of a mixed wall leaned toward defense. Will-O-Wisp and Shadow Ball will stay on its moveset. However, Rest and HP Water have got to go. Pain Split is a somewhat reliable way to recover Cofagrigus, with its low HP and all. Haze is for clearing the stat boosts of physical (or special) sweepers with stat boosts, like Houndoom or Weavile.
Yanmega is definitely my favorite UU special sweeper. He is great with Speed Boost. However, I will give you another suggestion after I rate him.
DRAGON!!...fly (Yanmega) @ Leftovers
Trait: Speed Boost
EVs: 4 HP / 252 SAtk / 252 Spd
- Air Slash
- Bug Buzz
- Psychic / Giga Drain
If you get one or two good protects in, Yanmega's speed will be nearly unmatched with 252 Spd EVs. Modest nature is more effective because after the speed boosts its speed will not need more investment. Psychic has better coverage with your team, but Giga Drain is good for when you switch in to stealth rock. 50% HP loss... urgh.
Here is my new suggestion:
Zoroark @ Choice Specs / Focus Sash
EVs: 4 HP / 252 SAtk / 252 Spd
- Dark Pulse
- Focus Blast
- Trick / Mirror Coat / Counter
Great Choice Specs sweeper. He has good coverage with your team, and can sometimes be a good counter to choice scarf Chandelure if it has focus sash and mirror coat. Also, it no longer has that nasty stealth rock weakness.
Hmm... I'm kind of on the bridge with this guy. I would go Jolly instead of Adamant with him. But here is another suggestion:
Froslass @ Leftovers
Trait: Cursed Body
EVs: 248 HP / 8 SDef / 252 Spd
- Ice Beam
- Destiny Bond
A Spikes LEAD. Froslass is great for quickly setting up Spikes and then using Destiny Bond at the end. Also, it serves as a great counter to other trap layers/leads via Taunt. It also provides a great spinblocker. Remember, if you use Froslass, it should be your lead.
OK, so this is what I think. Tell me if it works out! Good luck.
(Whew... This took a long time to write XD)