I've been using a Shellder on an LC team of mine lately, and it makes an excellent
late-game cleaner, as it plows through almost the whole tier except for maybe 2 or 3 pokemon with essentially two 125 BP moves. I use it with Scraggy, with one of them sweeping early-game to weaken the opponent and the other cleaning late game. It should also be noted that Shellder also gets Rapid Spin, which is an option for the last spot.
I can't believe how little votes Dwebble has. It is the fastest Shell Smasher and is quite a surprise when people expect a defensive set. Dwebble, along with Shellder, have the ability to break through Abra and Substitutes, and this is the reason I think they are better that other smashers.
I have fought against many Clamperl, I feel that it is the most used Shell Smasher. Even though it's Special Attack is through the roof with a DeepSeaTooth, it's defense is the lowest out of the five Shell Smashers and is easily revenged by priority moves. It can become almost invincible with Substitute, but then lacks coverage.
Tirtouga is quite the common smasher, and is great with Splash Plate Aqua Jet. Maybe I'm not seeing something everyone else is, but I don't like Tirtouga. It's the slowest Shell Smasher, and Mienfoo can break Sturdy and then Electric and Grass moves can take it out.
Omanyte is the only smasher I have never used. It seems like it's coverage would be great, and I want to know if it is as good as people say it is.
Abra can take a hit with a sash and KO any of the smashers. Dwebble and Shellder are the only ones capable of taking out Abra with multi-hit moves.
Shell Smash Tiers imo:
Also great thread Superpowerdude.