I like "balanced" speed combo, such as this one :
Morton + Landship + Slick + Cloud Glider
Good 4,5 speed with decent 3,25 acceleration is nice, with decent weight and handling.
Moreover, hidden stats are excellent, especially sea/air speed and mini-turbo (which is a +1 for Landship).
Another option is Morton + Circuit Special + Roller + Cloud Glider, which give a 4,75 speed and 3 acceleration. But rollers on a Formula 1 isn't looking great...

However the 5,75 speed combo is often the superior option, but it's much less reliable due to the 1,75 acceleration stat.
EDIT : just thought that acceleration tiers exists in MK8, so 4,75 speed/3 acceleration is better than 4,5 speed/3,25 acceleration anyway.
EDIT 2 : thinking about acceleration tiers, it might also be a good idea to go with Rosalina + Circuit Special + Slick + Cloud Glider to get in the 2 acceleration tier while keeping a nice 5,25 speed.
So best options are :
- In 1 acceleration tier : 5,75 speed (Morton/Circuit Special/Slick/CG)
- In 2 acceleration tier : 5,25 speed (Rosalina/Circuit Special/Slick/CG)
- In 3 acceleration tier : 4,75 speed (Morton/Circuit Special/Roller/CG)
- Over this, it isn't really worth it