So who did you take in Double Dash? For me it was all about going small, the heavy and even medium character just felt slower than the small characters. I usually took a baby and a toad, usually mixing it up.
Mario + Peach
Luigi + Daisy
Toad + Bowser Jr.
Koopa Troopa + Paratroopa
Petey Pirahna + King Boo
or Paratroopa + King Boo, Toad, Petey Pirahna or Toadette.