What's the best way to kill a floater in Dead Island?

I found that melee weapons work the best. Charge in, circle strafe around it and swing away with a melee weapon.

Or if you are able to knock them over, stomping on its head will also do it.

Rush in, keep at their back, and keep up a constant barrage, to kill a Floater.

You rush in because once you're in close enough, they'll feel ready to perform their harder-to-dodge, horizontal spray attack. If you move in slower, you're more likely to be in front of them when they begin a horizontal spray. An angle in front of them is a danger zone with the horizontal spray. When you're away, they're firing projectile...acid, which is easy to dodge (projectile hitbox is smaller than the projectile visual). Rush in before they begin a horizontal spray. I was going to say to keep strafing them, but if you're hitting them that means that they're flinching and won't be turning; you don't want to come 'round back to their front again.

When a Floater is struck, they usually get staggered. It's important to note that I say "usually". For example, as an exception, the Floater has "super armor frames", i.e. they will not flinch from being struck, when the Floater does a horizontal spray attack. I don't know if it's just a bit or absolute, i.e. 100% unflinching or if you hit them with a forceful enough weapon, they flinch. Point is, it may be better to dodge rather than counter-attack in the face of Floater spit, because they may not flinch when you expect them to, despite the advice of "keeping up a constant barrage" being based on the knowledge that they flinch.

It's not as practical, but if you knock them down, on top of the usual benefits to knocking something down, there is a chance that the Floater will clip through the ground and die. Cause of death is assumed to be by a death-zone underneath the intended ground. Floaters are notorious, at least in my experience, to die in this way.

"If you do a running leap kick, most of the time they get knocked down" - spartacus, What's the best way to kill a floater in Dead Island? .
This assertion is what mainly spurred me to respond to this question, because it is erroneous. Here's my video response to spartacus, partly since I cannot respond to their comment directly. The video demonstrates that Floaters cannot be knocked down by the jump-kick, tackle, nor knock-out attack. I don't believe that running affects the effectiveness of the jump-kick. Same with the knock-out attack. I demonstrate the variations anyway.

"The EASIEST possible way for killing the floaters is the homerun baseball bat that has near 100% crit attack everytime." - TwistedChaos, https://gaming.stackexchange.com/a/62037/46216 .
This assertion is only partially erroneous and employs exaggeration. It is not easy to acquire a homerun baseball bat. The crit. bonus of the Homerun baseball bat is listed as +75%.
Good news and bad news: Homerun baseball bats cannot be found, but are made. They are developer's craft weapon no. 2. Developer's craft weapons are made via the developer's craft blueprints, which require a two-step process to retrieve, unlike simply finding a blueprint. On top of that, each developer's craft requires 5 diamonds to create. Diamonds are rare.
As one of the Dead Island Wikis says, the main damage dealt by the Homerun baseball bat is not the strike itself, but the sudden stop on any obstacle toward which the target is flung. This method of dealing damage is in contrast to most other Impact weapons which deal much of their damage upon initial impact on a critical hit. Homerun baseball bat initial impact damage is poor because they only appear at level requirement 1 which means sub-30 base damage. Point is, even on crit., Homerun baseball bats do not always kill, instead sometimes merely setting the opponent back a ways.
"Like the other developer weapons, the price to maintain the HomeRun Baseball Bat is very high." - deadisland.wikia.com.

Floaters will not engage in uncontrollable emesis upon being struck with a poison crit..

Fire-arms: I am writing this in contrast to the top-voted answer's emphasis on melee. Fire-arms let you keep distance, safer from the Floater's attacks. A danger of the main tactic of constant assault, is running out of stamina. Fire-arms do not cost stamina. Floaters are bullet sponges, but they flinch from bullets all the same. Besides, since I'm usually using a Heavy Pistol and only pulling out a shotgun to insta-kill tougher stuff, it's good to have something to really unload an auto rifle on.