I just realized I wrote the wrong nature on Parasect, but since he doesn't seem to be worth having that doesn't matter, I don't think I can have much use of Snorlax as I'm pretty sure he's a OU Pokémon and I'm limiting myself to UU and NU. Could you suggest something that can be a secondary Trick Room user?
Oh right, forgot this was a UU team. Points stand, though.
Um, let me see. Slowbro would be a nice replacement to Slowking, since a good chunk of UU Pokemon are physical, last time I checked. Give him a moveset essentially similar to your Slowking right now, and maybe a few EVs in Defense. He can learn Trick Room as well.
Claydol is a good Rapid Spinner to clean out Stealth Rock and other entry hazards, plus a Trick Room user, although his speed is rather high for a Trick Room team. You could augment this with a Macho Brace hold item, which halves his speed. An Iron Ball with a Trick moveset could potentially work too, as it lets you destroy Flying-types and anything with Levitate, at the expense of your own Levitate (until you Trick it away), which is good as long as you don't mind the opponent potentially going first. You'd want to max his HP, then spread roughly equally among defense and special attack, maybe a bit more emphasis on defense. Rapid Spin is a must, and Trick if you're using an Iron Ball. Earth Power is an obligatory attack move, and you can choose between Knock Off, Toxic, Psychic, Shadow Ball, Stealth Rock, all kinds of options.
And Spiritomb is a nice option too, although he can't Trick Room. His speed is pathetic enough to make good use of Trick Room, and his typing is the best defensive one possible - zero weaknesses is too awesome to pass up. He can even Calm Mind to boost his special stats into insane levels, then level an entire team. Max HP is important, but the balance between special attack and defense is a bit trickier. Shadow Ball and Hidden Power Fighting have perfect neutral coverage, but since you'll be lucky to get that, you can try something else, maybe Dark Pulse. Pain Split brings healing along, and Will-O-Wisp helps Spiritomb deal with physical threats more easily.
Along similar lines (minus typing) is Hariyama, who has massive HP and strong attack, who can boost with Bulk Up to be a usable physical wall and attacker. He has the added bonus features of Guts, which is great if you can switch into a Will-O-Wisp or TWave, or Thick Fat, which gives him resistances to two common attack types. He can also Whirlwind to phaze setup sweepers. HP maxed on this thing is somewhat pointless, taking into account the fact that he easily tops 400 HP even without any EV investment, but a few points never hurt anyone. Attack maxed is important. You would be best off investing the rest into defense. Close Combat might destroy the point of the defense boosts, but after a couple Bulk Ups I dare you to find something that will survive. If I remember correctly, he can Belly Drum too. Hell hath no fury like a stomach-pounding sumo wrestler.
And finally, Steelix is an option. His defense is absolutely ballistic, and his attack, while below average, can be used with good old Curse. (There's an option! Forget Curselax, you got Curselix!) And it has the added bonus feature of boosting his defense, which can already top 500 with ease, to essentially unhittable levels. On top of that, he's slow as hell, and has no negative effects from Iron Ball, so his Gyro Ball becomes insanely powerful, even before taking STAB into account. And I know you really don't like Explosion, but after a couple Curses, NOTHING will survive. Besides, you can always switch Banette in to eat it without any negative effects. His big problem is that two of his common weaknesses, Fire and Water, often hit his very low special defense stat. Despite this, he should be able to crush pretty much anything. Max HP and Attack with a Brave nature. There's essentially no point in putting anything into defense that high. If you really want to, maybe take some out of attack and place them into defense.
These were all off the top of my head, but they could work.
By the way, the reason I don't have Explosion of Weezing is because I don't want to risk him taking another one on the team with him if I play a Double, and I was thinking of using Frisk in combination with Knock Off to remove the more necessary items from the opponent, rather then using it wildly.
Both sound points. I hadn't thought of that doubles thing, and Frisk and Knock Off both go well together.
But yeah, I'd like it if you suggested better natures, Ev's, movesets and such for those who you think need to change.
Yeah, sorry. Any nature that kills speed is good for a Trick Room team. Porygon2 might want a little bit more defense - maybe like 40 points out of special attack - but then again, after Tracing Intimidate, it'll be much easier to eat physical attacks. Anyway, boosting special attack is somewhat counterproductive, but I see it as more of a defensive Pokemon, so eh.
Slowking looks okay now that I think about it, since Weezing covers your physical walling needs. Calm Mind could probably go, maybe replace it with Nasty Plot, but then again I can see the special defense boost coming in handy. Psychic may get STAB, but overall it's a bad attack type; Ice Beam provides better coverage, and Surf already gives STAB.
Cacturne looks okay if you want to keep it. I have absolutely no clue why you'd put the last 4 into special attack; I know it's almost nothing, but placing it into defense or special defense might actually help in some situation.
Weezing looks fine.
Banette's fine. It learns Trick Room, so replacing something on there for another Trick Room user is a nice option.
And by the sounds of it you're not using Parasect, so. If you do, though, he looks good (well, as good as that mushroom crab thing can get).
As a side note, a Macho Brace could really go on anything here to help it become slower than as many opponents as possible.