I know what you meant, but the term filler is usually reserved for material that fluffs up a performance without adding much to it.
You're asking for Middlers, routines that go in the middle of your opener and closer. Or Body routines, if you're inclined to take terms from English Lit.
Asking what our favorite routines are is like asking who we rooted for in the Democratic Primary; you'll get the info you asked for, but it probably makes little difference to you.
Your opener and closer, if you've got them (and it sounds like you do), will tell you what your middlers should be. If you produce the four aces or whatever, use them in your next routine. If you produce a coin from a piece of flash paper, then you should consider a coin routine to follow it up with.
Remember this old theatrical saying from Anton Chekhov: "One must not put a loaded rifle on the stage if no one is thinking of firing it." If magic is indeed theater in its own right, you'd do well to keep this in mind.
Pj