On Sunday, I test drove the 2018 Kia Stinger at the LA Auto Show. It was so good, I compared it to my dad's Porsche Panamera (I learned that Kia also compares it to the Panamera, which is a bold statement). Very fast and fun, comfortable on crappy LA roads, and had a very high quality premium interior. It had a decent amount of space in the trunk like a Panamera. Don't have seconds thoughts because it is a Kia; they are making some really good cars now. The Kia will have, by far, the lowest maintenance costs comparing to the luxury brands. Journalists have been raving about it, and before you buy a car, you have to drive one. Kia has recently had a great reliability history, better than the German brands.
I haven't driven the MB, BMW, or Audi A5, but I prefer the A5 just because I've heard it is a better value and a better overall car. Plus, it looks sleek, especially for a four-door sportback. The interior is not as good as the Stinger after sitting in both.
I don't blame you for not buying a Jag, as their cars are not that competitive. I've owned one and my parents have owned many. They used to be more competitive.
The 2018 Buick Regal Sportback will obviously be the cheapest car on your list, and it'sperformance reflects that. I'm not saying it is a bad car, but you can't compare it to the other cars on the list. It is more of a luxury car than a sporty coupe like you sound like you want. It has a ride that is tuned more towards comfort than sport. IMO, it looks like a smaller Chevy Malibu in the rear quarters. On the bright side, the price of the base Buick is the same as a base Toyota Prius (a GS is $39k, which is like paying $39k for an upscale Malibu). Therefore, the interior will be more like a $28k Toyota Camry than an Audi or Kia Stinger. It has the same V6 engine as the Chevy Traverse and GMC Acadia family SUVs, engines tuned for efficiency, not responsiveness. It might make more financial sense if you buy a cheaper car or a used car since you won't be using this car for long. These cars depreciate quickly. Good luck!
Now I'm guessing NormT will say the Buick is better than all the other cars... I can read his mind.