1. Turret
The Turret does 125 DPS, dmg of 25 and fire rate of 0.2 secs. When compared to the Ranger, the time it takes it to deal 300 dmg, 3.5 seconds, the Turret can deal up to 437.5 dmg, 137.5+ more. The Turret is good at chewing through mini bosses and chipping a lot of health off a boss.
2. Ranger
The Ranger has a DPS of 85.71, dmg of 300 and fire rate of 3.5 secs. When compared to the Minigunner, the damage it deals in 3.5 secs, 280 damage, the Ranger can deal up to 300 dmg, 20+ more. The Ranger is good at picking off mini bosses and bosses overtime due to its long range and high dmg.
3. Minigunner
The Minigunner does 80 DPS, dmg of 4 and fire rate of 0.05 secs. The Minigunner is the cheapest of the three by a full upgrade cost of $18,100, while the Ranger and Turret have full upgrades costs of $44,000+. The Minigunner is good at chewing through the first zombies, making it a good front-line tower.
Note: I'm not including Golden Skins, Hardcore towers, or towers with high potential DPS but low output such as the Ace Pilot and Pursuit. Some of this information is false due to the RoF bug.