IMO, there's plenty of top-tier shotguns, especially when considering different applications. Here's some shotguns I feel are very worth having.
Tediore Blockhead - Probably best with the Gentle prefix, though New and Improved is also pretty great. 100% bonus splash damage is phenomenal, and the consistent shot pattern makes it very easy to use, not to mention the ricocheting cubes. This thing can keep up with a Ravager in damage with a quarter of the ammo consumption. The only thing preventing this from outright being the best shotgun in the game is that it's fire only, which means a good chunk of enemies have a 90% resistance to it.
Hyperion Conference Call - Yes, the seemingly disgraced super gun is still a fantastic weapon. The Interfacer has it beat fairly easily in terms of raw power, but the shot pattern on the Conference Call makes it excellent against large numbers of weaker enemies. Solid fire rate, pellet output, magazine size, low ammo consumption and excellent accuracy mean that the only downside to this shotgun is the somewhat low damage. Practicable or Critical are probably the preferred models.
Hyperion Butcher - It can fire for days at an unmatched fire rate. Salvador can fire a pair of these with every fire rate buff he has and he still won't run out of ammo with them. It has a low pellet count, which simply means it gets a proportionally bigger buff from the Practicable prefix.
Hyperion Interfacer - Like a high power Conference Call with slightly higher ammo consumption and a little less practicality.
Torgue Ravager - Casual, Sinewy, Juicy, they're all great. Shot for shot, this thing is hard to match. It's like an all-purpose Blockhead with twice the pellet count and majorly increased ammo consumption. The slower pellet speed and low accuracy mean that it's best used at close range unless it's in the hands of Zer0 or Gaige. Say goodbye to your ammo pool!
Jakobs Twister - I feel this one is kind of a niche weapon, but it's powerful enough to be worth mentioning. It has the highest raw damage on the card of any shotgun, but its projectile speed and shot pattern make it best used against large and/or slow enemies. Once again, Zer0 and Gaige can somewhat get around this.