It's cliche and overused but /Dark really is a strong secondary (my first villain 50 was a Bots/Dark mastermind) good heal, lots of nice target debuffs to help your pets, some nice control powers and even another pet at the end.
I found /Poison good if your going up against a lot of single higher targets, not so good for large groups.
Haven't played with /Rad yet though I imagine it's probably pretty strong.