What car would be the best for me?

2 Days ago i was hit from behind by a drunk driver totaling my 2007 ford mustang gt. I had some work done on the car and it was my baby. Now i'm looking for another car and I'm having trouble deciding what to get. I want something that is better on gas and kinda inexpensive to supe up. I have about $20,000 to spend. I would be really grateful if i could narrow down my choices.
Cars i've been looking at:
-Mustang v6
-Camaro v6
-dodge challenger v6
-hyundai gensis 2.0t coupe
-jetta GLI
-scion frs
-subaru brz
-subaru impreza wrx sti (can't find any cheap enough)
- mazda speed 3
-golf gti

I would appreciate any information on these cars. Really need help finding something thats still quick but better on gas than my v8 was.

Not true a 2011 v6 mustang makes more horsepower than a 1999-2004 mustang gt and it almost as fast as the gt mustang. It also has 6 speed.

So if the v6 muscle cars arent the best bet, even tho they do have 300hp. What should i go with besides the gt because i don't want to have to continue to spend so much money of gas

Gotta agree with jase. After owning a gt, getting a v6 would be like getting a lapdance when your married from your wife and it only costing $1

Get what your budget supports. If fuel costs are a concern, naturally you aim for a 4 or 6 cylinder. If V8 power is a requirement, well you know the costs associated with that. Just a matter of where you want to spend your money. As far as modifications, you're not going to save money there no matter what you choose. It costs money to go fast no matter how many cylinders you have. And ignore the V8 peer pressure. Unless they're going to volunteer to pick up the difference in fuel costs.