I saw someone put 700 as the best, but I think that 400 is the best. With my testing of 700, I noted the following few anomalies:
1. The gimbal whigged out on me like it was straining but this only happened a couple times and was fairly rare. I'm sure if it was allowed to continue, it would burn out the brushless motors.
2. After 400, I believe they they added another data channel that can be used to intercept drone information by a 3rd party while flying. This has resulted in reduced bandwidth for your remote from the drone and subsequently resulted in reduced range. I have far better link to the aircraft with 400 over 700 and with 700, it was the only time I had a low data connection warning with the aircraft flying in the same spot I fly 400 without issue all the time.
3. They added all the NFZ garbage after 400 that has led to the drone refusing to take off saying it is in a NFZ until it gets a solid GPS lock. This has resulted in at least one drone crashing into the ocean after the app crashed. 400 doesn't have this garbage. This is not as much an issue anymore after you figure out how to mod the firmware, but still an issue if you don't.
4. The horizon tilt on 700 that needs to be corrected for in app is not present on 400. This horizon tilt gets worse if you go to portrait mode. 400 has no issues with either mode.
Some positives of 400:
1. It is noticeably faster in response.
2. I believe its top speed is faster than later revs.
3. Braking is much improved.
All of these are not such a big deal after you figure out how to mod your firmware.
One negative of 400:
1. It will slide a bit to the right when coming to a hover from speed. Not a big deal, but plan a bit for it. I don't think 700 did this to me.