Rules Systems

Not sure what rules system I want to use. I'm most familiar with 3rd Edition, but I'd like to give an earlier edition or retro-clone a shot. I've also considered designing my own rpg, but I haven't gotten very far and it'd likely be so similar to D&D anyway it might not be worth it. I've also been working on a modification of 3e which is a lot more likely to get done. I also want to DM 4e, but if I do it'll likely be in the second campaign I have yet to describe.