Dawn: 1-40 is faster than normal, 40-100 is slower than normal.
Midday: Normal leveling pace.
Dusk: 1-40 is slower than normal, 40-100 is faster than normal.
There are more benefits to dusk though, in that it will take 10,000~ less exp to hit 100 than dawn.
Edit: If you are going for a speed run, pick dawn.
If you plan on completing the game 100% (Or plan on going to lv 100) pick dusk.
Midday I guess for just normal runs where you aren't looking for anything specific.