Shaymin-S should not get banned unless we decide "It's too haxy" or "it's annoying" are legitimate reasons to ban something (at which point I'd be cool with banning it)-- because its power is nowhere near "top OU."
Leaving out the rest of your post because I don't really care about the other suspects as much as I care about Skymin.
Not sure exactly what metagame you're playing in where Skymin isn't near the top of OU but I, and many others, think that it's by far the best Pokemon currently in OU (and broken beyond belief).
Its speed still remains near the top seeing as most new threats this gen have settled in around the ~350 area meaning it's still packing amazing speed, great special attack and an amazing special movepool. Combine that with a powerful air slash that flinches 60% of the time and a STAB 120 power move that lowers special defense by 2 stages 80% of the time and you're looking at a pretty ridiculous Pokemon. It's not "haxy" when on a turn by turn basis it's likely to get what it needs.
Then if you look at what it can do it has 3 main sets really: Life Orb, Scarf and Leech Seed.
Life Orb hits ridiculously hard to a degree that reminds me of trying to switch in on Mixmence. The only difference of course is that if you make the right switchin on Skymin you still have a high chance of losing because of a special down or flinches. Since it's so fast it's hard to switchin anything faster that can take the hit and then immediately threaten it, you're pretty limited in how to deal with it. It can run through both offensive and defensive teams.
Scarf is an awesome revenge killer with ridiculous speed that still hits hard as fuck. Any Pokemon that fast with a 60% flinch move is going to be a problem to get by (even Jirachi who had to rely on a shitty steel type move was hard to get by at some times). It really runs straight through offensive teams.
SubSeed is just a complete bitch. Bologo outlines pretty well what makes it so good compared to other subseeders. If you get a free turn with this thing against a defensive team your opponent is going to be sent scrambling trying to figure out just what the fuck to do to take you down short of pp stalling you. I've been on both ends of this.
So really, no matter what playstyle you use there's a Skymin set out there that is going to give you massive trouble and the Life Orb set can just decimate all styles (stall has an easier time i guess because of sand stream and a few protects here and there but Skymin usually takes a huge chunk out of the team before it goes down even if it often predicts wrong).
There's just nothing out there right that compares to the speed, power and versatility of Skymin (or if they do they die to an unboosted mach punch).