I personally use Lakitu, Bottle Rocket, Slim, and Swooper for everything. With high acceleration and handling, it recovers almost instantly from most hits and is very easy to control. The problem with karts that max out speed is that they take forever to get going and can't make good use of their "maxed" speed since they tend to corner poorly and are stopped by nearly any hit. Using this kart, I rarely finish below second, both when playing online and when playing the standard GPs. It got me to an all 3 star rank and about 4000 VR in less than a week and a half of light playing (less than 18 hours total play).
Anothre kart I found that works well is the only set that gives my first set trouble on courses like Dino Dino Jungle, Maple Treeway, and occasionaly Kalamari Desert. The combination is Peach, Egg1(either that or Soda Jet, IDK which), Red Monster, and Peach Parasol. This cart can drift through grass and maintain/acquire a drift boost and barely slows down off road. It may not have the best speed, but it recovers fairly quickly from hits and gets back up to top speed just as fast. It tends to hang around 2rd to 4th on the first couple laps, ending in high position on the last lap and usually finishes first with proper item useage.
If anyone wants to see the kart in action, just PM me and we can arrange a wifi race where I can demonstate.
I'll be racing on wifi for the next while anyway. If you encounter someone using a Mii that looks like Brock (the Kanto Gym leader), that's probably me.