From worst to best:
The battle royale/circle of fire is way too snowbally. There's no way to opt out of this event, like the other ones, which you can just cede over in exchange for something else on the map. And if you lose the event, the opponent doesn't just get the event reward, they also get a teamfight win. Not to mention the random placement of the circle which sometimes decides who wins the event.
The Teemo-killing event is just a Baron-esque smite fight. Not really interesting, IMO. The best strategy seems counter-intuitive: Ignore it until the enemy wastes all their spells on it or gets put into a bad spot due to chasing it. And again, the random wandering can have a pretty big impact on who wins the event. (Edit: Just realized that it gives gold when you auto-attack it. So it's better than I thought, but still a bit wonky.)
King of the Hill is fine, except for the random placement. But at least the patch limited how unlucky the positioning can be.
Payload pushing is like King of the Hill. Except there's no unfair positioning, but with the addition of an extra reward (a tower kill). And it's slightly skewed in the favor of the team who's behind, because they have a closer target, assuming their mid tower is already dead and the opponents' isn't. That small come-back mechanic is nice I think, as long as it's not too severe. (Jury is still out on that.)
The Final Nexus fight is pretty hype and a good way to end the game. It's not too crazy; usually you just teamfight and then kill the nexus, but I think it does it's job well. The Nexus abilities are a nice touch, although the Nunu ult seems a little strong.
And snowball fight is pretty great in general. There's a lot of good play around the lanes with it: People spread out between both lanes to get better/different angles, minions can be hid behind (which also makes waveclear an important mini-objective), and the winner isn't really dependent on who's ahead/better at teamfighting like the other events.
As for the rewards, they're alright. Nothing to write home about.
The big shield can be a bit frustrating if the damage it blocks ends up being irrelevant (for example, if you were just going to heal up all the damage anyways).
Cursed minions are just weird, and they can actually be a detriment if it means you get less CS. But at least they're interesting (to me).
There's not much to say about the GA; it's decent.
And the Catapult is pretty fun, but me and a lot of others keep forgetting to use it :\. It may also be OP, just because it can last the whole game.
I hope I didn't miss anything... Are there really only 4 rewards right now?
(Edit: I forgot the Ocean buff. It's probably the least interesting reward, and yet quite powerful.)