Before the 1.3.0 update, I would say Ink Armor and the Inkjet were the top specials in the game. Ink Armor was pretty diffiuclt to deal with, and can be chained to almost always a win in a teamfight against a tream who didn't have a lot of Ink Armor weapons (which was part of the reason why the Tri-Slosher, a long with other Ink Armor carriers like the N-Zap 85 and Splattershot Jr were so popular at that time).
For the Inkjet before the 1.3.0 update, it was a very solid special and can be down right devastating in the right hands and the weapons the special was on were very good as well (like Tentatek Splattershot, Octobrush, and Custom Blaster). On top of that, Inkjet had good synergy with Ink Armor making it pretty difficult for most weapons to take an Ink Amored Inkjet down.
As for the worst specials before the 1.3.0 update, from what I've read and heard from various players it was the Stingray and the Baller (before the 1.2.0 update).
After the 1.3.0 update, Ink Armor seem fell out of favor a bit (it's still a decent special but not necessary for a team comp) since the nerfs and I honestly feel like Inkjet got overnerfed and some players seemed to had stopped using weapons with the special (some dropped the Tentatek Splattershot for the Vanilla Splattershot).
For the Baller and Stingray after the 1.3.0 update,
the tide has turned for them. The Stingray is no longer "the worst special" and it's a pretty good long ranged special that does particularly well in Tower Control or on curtain maps like Port similar to the Killer Wail special in Splat 1. The Baller (imo) is currently the best special right now, the buffs made it pretty strong and many folks who scrim had taken a liking to it in their team comps as of lately (even to the point where the Areospray RG is viable since you can get the Baller special in 6-7 seconds at the start of the game with some special charge up). I feel like the increase to it's blast radius and the increased minimum damage caused by the explosion is what really made higher level players start using weapons with the Baller special (like the L-3, .52 Gal, and Areospray RG).
Every other special is somewhere in between Bomb Rush, Splashdown, Tenta Missiles, and Ink Storm are all strong specials and do a good job at what they do (I don't have much of an opinion of the Bubble Blower special due to my lack of experience with it) .