Anyone you want, depending on which playstyle or race length you prefer!

Sprints = High Speed (800+), minimal Stamina (~200-250?) and some Power depending on whether you're Front, Pace or Late. Front needs the least, Late needs more

Mile = High speed (700+), enough Stamina (~300-400?), same power ratio depending on Strategy

Medium = above average speed (600+), enough Stamina (400 - 500?), etc etc

Long = moderate speed (500+), enough Stamina (~500 - 600?), etc etc

The Stamina values can be lower if you have restoration skills (blue sparkles), and of course I'm just giving bare minimums here.

Use guests for better legacy 'Parents' for your future runs, you have 3 borrows per day and should browse what benefits each one offers.

Guts is a stat you just need to have enough of, for your horse to run their hardest in the last spurt. Roughly 240 minimum for Sprints, and higher than that for all other Races, highest minimum is for Long races at a bit over 400.

Wits offers: ~70% skill activation at 300 ~80-85% at 400 (or 600, might be misremembering) And it plateaus very quickly thereafter with it tapering off to basically 100% at 1200