if you're talking about sheer clutch factor, i'd say the inkstrike takes the cake quite easily. being able to get guaranteed turf coverage (unless any nearby opponents are savvy enough to shoot into them!) and neutralize / capture splat zones from a reasonable distance is simply amazing. on top of all this, you can use it to flush enemies out of their nests (such as the ones easily made in moray towers) and create a bit of a smokescreen to make approaching your opponents a much less daunting task. while you do have to take yourself out of the game for ~2-3 seconds to launch it, the benefits gained from launching the inkstrike very much validate the time loss. the only mode where i'd say the inkstrike is not as valuable is in tower control, and even then, you can use a smokescreen-esque application to help protect teammates on the tower or force opponents off it.
bubblers are fun to use and incredibly dangerous in the hands of a skilled player. if said skilled player happens to have equally skilled teammates, then your best option is to just run away and try to stall out the bubbler's time limit before your opponents find and splat you. the only drawback is that you can shoot at them to physically force them back, but it really isn't that bad. the kraken isn't too bad; the fact that you never have to worry about being caught in your opponent's ink makes it ideal for (re-)capturing splat zones and riding the tower in tower control for the last stretch until it hits the goal point.
that being said, my personal favorite special (and the one i'm most wary of whenever it's used by any opponent) is the echolocator. being able to see where your opponents are for ~12 seconds is quite a big deal, considering how critical the element of surprise is in this game. just with the click of a stick, you and your teammates have an effective immunity to ambushes and flanking attempts, given that your teammates are somewhat competent at the game. echolocators also force a more defensive playstyle on opponents, unless they want to get splatted. on top of all that, the echolocator provides an instantaneous ink tank refill courtesy of having no activation animation, which effectively gives its user the ability to keep spreading ink without having to retreat into squid form for longer periods of time. while cold-blooded makes this special much less useful, it's not that common a sight among most players i've seen...yet. as the splatoon meta develops further, i think more players are going to realize the utility behind cold-blooded in keeping echolocators (as well as point sensors and haunt!) off their tail.