You really like them swaps don't you...

My non-player opinion would be B~A>C>D

I can't think of any reasonable use case for D, and C feels more like a disruptive challenge piece (like ChaosPortal but worse) than anything useful. A and B feel like they'll have their own use case over another (eg ice>poison if you want to target the swapped unit without letting it move freely for 3 more turns), though value wise B would probably be higher by a bit.