Depends on your goals. If you want to farm datacores forever, then of course the corps with the most datacore agents. If you want to run combat missions, then corps with a high number of combat-oriented agents. If you want to maximize the LP returns, without worrying about maintaining neutrality with other factions, nothing can beat the Thukker tribe (for the nomad implants).

Also note that if you pick the best NPC corp possible, you can be sure that full time missioners would also have chosen the same corp, and so your isk/LP for that corp will be lower. In the long term, this does make a huge difference, as, for the higher level missions, the LP is a big (>50%) part of the reward scheme. So picking a not-so-popular missioning corp might work to your advantage.

The way to approach the solution for you would be to figure out what faction(s) you want to run missions for, then to decide which region of space you want to operate in. Once that is done, fire up http://www.eve-agents.com and find out systems with the most high quality L4 agents nearby (within 3-5 jumps).

An additional factor would be finding L4 high quality agents in systems that are far from any low sec space, since these days many more L4 missions require you to visit Low sec to do them, even if the agent is in high sec.

Good luck!