Good Question OP.
There are so many variables and there a lot of guns I haven't shot. You didn't restrict the question to self defense guns, so I'm sure the fastest would be a 22 or a 9mm race gun.
If you take 22s and race guns out of the question, I would have to say my CZ75 P-01. I had an SR9c at one time, and it stayed on line with very little flip. The Walther P99AS has a very short reset (as does the PPQ) and they are very easy to shoot quickly. Once you adjust to their slightly higher bore axis, they also shoot accurately. My son's Glock 19 is also very good at follow-up shots. The G17 would probably be even better.
No matter which gun you shoot fastest, you will shoot it faster and more accurately with good practice sessions and remembering the adage, "slow is smooth, smooth is fast"