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.

The Sniper and its upgrades will have the most bullet Speed, with the important trait

The Sniper and its upgrades’ Bullets do not slow down when travelling through obstacles (enemy Bullets, Polygons), which may be helpful for farming. From another side, a too high Bullet Speed might be the reason the Bullet will fail to deal the max amount of damage to small and strong Polygons like the Pentagon.

In addition, the Sniper gets a natural increase to bullet speed.

The Sniper has an increased field of vision, as well as increased Bullet Speed, Damage, and Penetration.

Upgrading into Assassin for more Bullet Speed

Upon upgrading, you receive:
Increased field of view
Increased Bullet Speed
Decreased Reload (rate of fire)

And finishing with the Ranger

Upgrading from the Assassin to the Ranger passively increases Field of View, Bullet Speed and Bullet Penetration while decreasing Reload and Movement Speed

Empirically, I have done some testing in the sandbox and the Ranger shots cross the map in roughly 2.84 seconds, while the Predator Tank takes closer to 3.52 seconds, even when zoomed.