Until now I used the Deathlock Wight, which was really fantastic when it used its reanimate ability for the first time and the players where like "WTH".
The corruption corpse was really effective from the roleplaing & horror point of view: When describing them how it teared its rotten flesh from its body and throws it at them, you should have seen the look on their faces.
And finally I used the Beholder Eye of flame in a battle with another "normal" and some yugoloth in a fight versus level 16 PCs, and still the Eye of Flame contributed to the fight with its supporting abilities, which was really cool.
I would love to see some of the other monsters converted, as I stated in a former post, the Draegloth Abomination and Iron Golem Juggernaut are in my main interest, but I am happy about every other converted creature I could use in my play!