It depends on if you want to satisfy a short term goal or long term goal.
A short term goal might be wealthy at level 40 to be able to get and buy that level 40 mount fast. Professions that makes armors/weapons would be good for this. I find Alchemy and Enchanting bad for this as nobody buys potions/flasks at from level 1 to 40… even enchants. Enchanter has the lowest gold income of all on leveling as all the greens you see would be disenchanted.
A long term goal might be wealthy at max level. This is when potions/flasks would be hot in AH. Also, all the glowing enchants would be needed at max level for raiding. Alchemists and Enchanters would be rich at max level.
If you have time, I recommend you to have Making Armor professions on leveling. When you get the level 60 mount, reset your profession to Alchemy or Enchanting.