Whatever TMNT they make I'd more than likely get. I love the art direction of this line, and it's my favorite franchise. My top 5 would be.
Shredder
Casey Jones
Bebop and Rocksteady (counting them as one also)
Slash
Usagi Yojimbo (not sure if possible)
Pretty much everyone that's been listed would be instabuy. I'd realign my whole collection for these