I love Rotom-W. With insane special stats and defense, this guy is the perfect wall. With his only weakness being the uncommon grass type (not including Mold Breaker EQ's) this guy is always seen on OU teams. With already good enough SP D, Will O Wisp helps this guy out a lot more by crippling physical attackers. He is also a good Trick+Choice Scarf user.
He is, however, is countered by a Mold Breaker Mega Gyrados with Earthquake, and is also countered by Excadrill as well. If you are using one of these two Pokemon against Rotom W, it would be best to attack straight away before it can manage to burn you.