My son seems to thank that the Rat King is the best because that is where he stays, then he has all the turtles, Splinter, April, Leatherhead, Shredder and the rest of the bad guys come and "visit"
The Four Basic Turtle Figures and Splinter, You can add any number of the other figures to stimulate interaction.
The regular sized ones, not the jumbo turtles. They are several in the regular size because some talk or have certain actions. Any of them would be fine.