I find shifter to be my go to class. I really enjoy the fact that I do not have to be super gear dependant. I will admit that I have not tried all the different types, but many of them have some one who should be able to explain it to you. I personally like Def/Off.(See Tinwold)
Pachyderm is amazing in PK, I could survive Greedor and Arratok's entwines/flurry and what not with it. Alligator gave some fantastic damage as well as regen and amphibious ability.
I would love to try Ut/Ut just to see how they handle.
My suggestion is to rank fast. Get that first form asap and go from there. Enjoy the ride, I like the excitement of waiting to see my next form and talking with others about which forms we really want.