Those ding dong funky flamers....

Real good combos:
Baby Daisy/Falcon
Baby Luigi/Cheep Charger
Toadette or Dry Bones/Mini Beast
Baby Mario/Standard
Toad/Tiny Titan
Baby Luigi/Bullet Bike
Baby Mario or Baby Luigi/Jet bubble
Baby Luigi/Magikruiser
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Peach/Dolphin
Peach/Sprinter
Yoshi/Sneakster
Luigi/Mach
Daisy/Classic
Luigi/Wild Wing
Yoshi/Standard
Diddy Kong/Blooper
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Waluigi/Flame Flyer
Funky Kong/Honey Coupe
Dry Bowser/Jetsetter
Waluigi/Offroader
Waluigi/Piranha
Dry Bowser/Standard
Waluigi/Flamer
Donkey Kong/Shooting Star(The only outside drifter, btw)
Dry Bowser/Spear
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There you go, the best combos.
Personally, I like the Spear the best. Even on Rainbow Road, it works for me.