Being a almost pure crafter who gathers all of his own materials I'm somewhat limited. I have my fire beetle which I trained up as best I could
Normal: Cows and bulls, Delucia and around is great for that. Good spawn rate and little to no risk, Fel for that double resources if you're on such a shard.
Spined: rat men in Ice, slow enough spawn rate I can keep my bug healthy and quick enough that I'm not waiting for them to spawn. Again, Fel if you're on such a shard.
Horned: gets tricky with a low HP bug but when I'm feeling brave I'll hit wyverns near the gypsy camps in the desert in Ilshenar, cant remember the correct name for the area though.
Barbed: another tricky one with the low HP bug but I try for the wandering nightmares to the far east of Umbra. This is difficult due to the other random spawn.
There are much better areas to farm if you have the skill set, the lizardmen at The Sleeping Dragon champ spawn for example. If you're quick at skinning, any area affect spell and you'll fill up your pack with spined in no time. It all comes down to your play style and the amount of time you want to put into gathering.