The ID that is best is literally the ID that YOU PERSONALLY want to use to hunt with.
If you're referring to which IDs are the best for hunting gear for that specific character, I can make some recommendations:
Hunter: Skyly, maybe Oran
Skyly is always nice because it has a chance of dropping the illusive Sealed J Sword. Oran isn't too bad, but overall I would say Skyly is better. (Personally I made my Oran Hunter a HUmar who I play far less often that my Skyly HUcast).
Ranger: Purplenum, Whitill, or Greenill
Purplenum is an awesome ID to have for a Ranger. As Midori said, Purplenum gets the most event drops out of all of the IDs. It also lets you snag Frozen Shooter really easily from Hildetorrs in F2 (1/2 drop rate) as well as hunt Spread Needle (Merlan 1/900) and Bringer's Arm (1/358) from Ruins. You can also use Purple to hunt Psycho Wand from Mil Lillies in Caves. Greenill has most of the same drops as Purple, but it's nice to have Greenill to hunt materials for your various characters. Whitill is overall a good ID, and as Mud said it is by far the easier ID to hunt Red Rings with.
Force: Whitill or Pinkal
Again, Whitill is just a good ID to have. It gets overal nice drops and is the easiest ID to hunt Red Ring with. Pinkal is not an amazing ID, but it does supposedly increase the rate of technique drops, which you probably want for a Force. It also let's you hunt MG+, an awesome Armor piece to have since it boosts the power of Grants from Dorphon in Ep 4.
Overall: Purplenum and Whitill
Hope that helps you out a little bit.