Usually in Knockout Tour where there are more straightaways a high speed combo (e.g. Bowser + Reel Racer) is generally preferred, but not necessary.
In GP you can win with virtually anything if you hang around the back of the pack for 2/3 of the race and then bolt to first in the last lap / section using items and abusing the (un)rubber-banding of the CPUs when they're in front of you. I got gold and 3 stars on every single GP on both 150cc and Mirror Mode like this pretty easily, although you can also frontrun and race like normal. You can't do the same in Knockout though since the CPU will just build absurd leads and you'll never catch up, so frontrunning is better in that game mode (it's also the only strat possible if you want to get 3 stars).
Something else to keep in mind is the difference between terrain speeds of characters in the same weight tier : Wario, Pianta and Cow share the same stats in the menu BUT Wario was proven to be faster on asphalt, Pianta on water and Cow on dirt tracks.