What tanks have the most FOV?

Solution 1:

From the Diep.io Wikia page on Stats:

1. Ranger
2. Assassin, Stalker (this tier is slightly bigger than the Hunter tank)
3. Hunter, Streamliner, Predator*
4. Tied between different branches:
   • Sniper
   • Overseer branch (Overseer, Overlord, Necromancer, Manager, Overtrapper, Battleship, and Factory)
   • Trapper branch (Trapper, Tri-Trapper, Gunner Trapper, Mega Trapper, Overtrapper, and Auto Trapper)
   • Smasher branch (Smasher, Landmine, Auto Smasher, and Spike)
5. All other tanks.

The Predator has a special ability that allows the camera to move away from the tank, allowing it to see further than its normal FOV could hold. Not all is visible at once, however.

Conclusion

If we use raw FOV (without camera-shifting), you're looking for the Ranger.

Including camera-shifting, the Predator has the most FOV, as you can see almost one screen more in every direction, although not all at once.