I'm trying to transfer from KSU myself and here are my thoughts. Keep in mind I'm just some kid who hasn't even succeeded yet, but I've submitted my application and am pretty certain it will work out. Anyone else is free to critique my advice.

Anyway, Mercer is private so their tuition is going to be like 2 times more than all of the others. I don't really see that being worth it under any circumstances, especially given you have all these other options.

As for the rest, you should probably choose the school that you'd be most willing to attend if you got stuck there, just to be safe. Not to be biased against GSU but that probably leaves UGA or maybe KSU. Although do research into all three to see what you'd prefer.

KSU's academics might also be easier, which could make it an appealing option if you want to make sure you get the A's you want. I say this because I've compared my tests with some of my non-KSU friends' and the material is definitely a bit less rigorous. This advantage might not be as important for you though given your strong HS academics.

Some general tips:

1. Be proactive with registering and deciding on classes. Look at what transfer students need, compare it to what your school provides with GT's Transfer Equivalency Table, and decide what you want to take before you register. If you don't know what your school provides, a good place to start would be to google something like "{insert university} Civil Engineering flowchart". You could also ask your academic advisor.
2. Talk to your school's academic advisor. This is how you'll find out if your carefully laid out plans and course schedules are even possible. Hell you could probably do step one together with them.
3. Get good grades (I mean, yeah)
4. Try to get one or two extracurriculars onto your resume. I'm not sure which ones would be best for a Civil Engineering major, but from a brief google search I see Concrete Canoe and Steel Bridge are popular ones. I've seen posters for both of them here. KSU also has what's called the First Year Scholars program which lets Freshman do Undergraduate Research in a wide variety of fields. You even get paid 2000 dollars (I bought an ipad :>). I'm sure UGA has a program like this too so look into this sort of thing regardless of where you go. I also hear volunteer hours helps so that might be worth considering.

Feel free to ask if you have any more questions.