Due to time restrictions can only do one part of the tour, which would be the best?
Do you mean only one of three tours available? If so, personally, my favorite was #2. If you meant if you can stop halfway on a tour and leave, you probably can, but it’ll be hard to get back tp your car because you are on their bus and they travel a lot.
I've heard that Zombie Tour 3 is actually the most interesting tour -- you visit more locations. But I really liked Zombie Tour 1 as our tour guide was really good. Ask for Charlie, aka "Chaz", aka the zombie that Daryl & Rick gutted open in Season 2's premiere.
If you can only do half of Zombie Tour 1, maybe join in for the latter half? That way you get to see the Atlanta skyline, made famous by the official TWD poster. I believe we also got to see Terminus (from above) before that. What I enjoyed was behind the scenes info and clips from the tour guide to anchor you at each location.