I suppose Electro Bolt has to be the "most useful" plasmid. It stuns enemies, it disables machines, it opens doors and it turns water into a seething pool of electric death. This is the first plasmid you get and it is the one Plasmid that I never swapped out over the full course of the game. In fact I became so dependent on the ability to stun opponents that I really struggled when enemies with electric immunity popped up towards the end of the game. Most useful doesn't necessarily mean most fun though so lets consider some others.
Telekinesis, the ability to move stuff around has a lot of potential for fun particularly with the ability to modify the environment and use assorted items of junk as traps and weapons. It is also great for returning flaming missiles to their originators. In order to get the most out of telekenesis though you would have to work at it and I didn't spend much time with it. It just isn't my play style I suppose.
Incinerate certainly creates fireworks and it is amazing how many things in this game can be set alight. I was disappointed by how little damage incineration seems to do though. This might have been better if I took the appropriate gene tonics to maximise fire damage but I didn't.
Winter Blast is the the chilly counterpart to incinerate. I never got to like this. It always seemed to be a desperate race against time to try and shatter the frozen enemy before they thawed and even if you were successful shattered corpses give no loot.
Enrage turns a hostile mob into an ally. I used this skill early on when facing groups of enemies but I grew to dislike it after a few failed attempts. The problem is that if there are no other targets in sight the enraged enemy will turn on you. That doesn't work well with a stealthy playstyle. You are more or less forced to confront mobs en masse in order to use it successfully.
Hypnotise Big Daddy is a great plasmid. These guys really kick ass and I got a great buzz out of having my own personal Frankenstein monster to fight by my side. Unlike Enrage there are no bad side effects. The hypnotised Big Dad won't turn on you if there are no other targets around and won't turn on you when the effect wears off. Unfortunately when a Big Daddy stomps around the earth literally moves. The whole screen jumps up and down with every step. At first this seems like a cool effect but it becomes tedious if not downright nausea inducing if you have to endure it for any length of time.
Security Bullseye makes the recipient a target for the security system in your place. Meh ...I never really used it. I preferred to hack security devices myself to achieve the same effect.
Cyclone Trap allows you to lay tornado traps that throw enemies into the air and dash them to the ground. I never used this. I guess trapping just isn't my style.
OK enough beating around the bush. The time has come to reveal my all time favourite plasmid. And the winner is...
Insect Swarm: A swarm of bees flies out of your hand and stings the daylights of any enemy that happens to be in their path. At first I thought this was just a novelty but then I realised that in the right circumstances the swarm can do a huge amount of damage to a group of enemies, more I think than a single shot of any other plasmid or weapon. Part of the joy of using bees though is that its affects aren't always predictable. The swarm doesn't shoot out like a bullet towards its target. Instead it seems to meander. Sometimes an enemy will avoid the heart of the swarm and endure only one or two stings. At other times the enemy will get stuck in the middle of the angry swarm and dance like a swatting lunatic as the angry bees slowly whittle their life down to zero. Beware though lest a stray bee sting an unwanted target. Even one bee sting is enough to send a Big Daddy tearing after you in revenge. To me this unpredictability is part of the joy of using insect swarm. One of my favourite tactics was to sneak up to a group, release a swarm and then leg it before the first sting alerted the enemies. From my secure hiding place I waited for the buzzing and accompanying shrieks to die down before going out to inspect the damage. I have even killed Big Daddies this way although it takes a few swarms and a bit of stealthy patience.