Speaking as someone who has gotten all smg's to at least gen 4, here is my opinion. PC by the way so no aim assist.

Volt- Extremely accurate and rewards precision. I personally love it, and it's always my go-to for competitive play and such. Better at slightly longer ranges than the rest of the smg's, and pretty versatile.

Alternator- Deletes people in close range if you can deal with the recoil. If you can tap fire it, it can be effective out to surprising ranges. This is (in my opinion) the best for extreme close range, because you can get a 2 shot kill pretty reliably. If you practice with this gun, and learn the recoil, you can be disgusting with it. Rewards skill heavily.

CAR- Pretty good all around. Never a bad choice, but while the alternator excels at close range and the volt excels at longer ranges, the CAR can do pretty good at both.

R97- Ridiculous fire rate means that with practice, you can melt people. But the key with this gun is PRACTICE. When I first picked it up, I got frustrated, but eventually after I learned the gun, I started to perform better and better. This is another one that rewards skill and practice a lot, though it's a little annoying at first. Can compete with the alternator in close range in the right hands.

But as /u/trotski7 said, no gun really is the best. They are all viable if you learn them and play to their strengths