That's actually a very tough question, because in my opinion none of them are terrible/unusable. Some are definitely more situational than others, but most skilled players can make solid use out of each one in different scenarios. Ink Mines and Sprinklers are maybe the most situational subs, but in the scenarios where they're useful they can be very useful. Ink Mines are a huge nuisance in Tower Control, and can actually be difficult to flee from when used as a Super Jump kill parting gift. Sprinklers, meanwhile, can be great for passive pressure, distraction and meter building. The enemy is forced to deal with them if they're tossed on key areas, which you can't say about many subs.
As for worst Special, I'm really surprised by all the Inkstrike hate. Yeah, it's probably useful in fewer situations depending on the mode, but it is honestly the best Special, period, in both Splat Zones and Turf Wars. It is also the ONLY Special that can be reliably used to push an obnoxious sniper off their perch, instantly burst a Rainmaker bubble (aside from Killer Wail), or provide a huge wall of pressure/protection to a tower from anywhere on the map.
For ages I actually thought Echolocator to be the least useful Special, but man oh man has that opinion changed. It gives huge benefit to virtually every attack strategy in the game. It literally hands you on a silver platter exactly where to go for any given flank/snipe/pressure/defense. When it's utilized properly it's so horrifying to deal with. That being said, it's a lame pairing with certain weapons. How on earth anyone thought it would give much benefit to the incredibly short range Tri-Slosher, I'll never know. But the benefit it provides teammates is almost always welcome.
Bomb Rush and Killer Wail are also what I would consider situationally useful, but again, when they shine they really shine. Bomb Rush can cause devastation to zones, while Killer Wails can completely clear a tower's path. It's really a matter of knowing when and where to use them. Once you've got that down, I wouldn't say any one sub/Special is that bad.