I've been rewording this a bit as I typed so bear with me :)
Jumpman, to be fair, I was only disagreeing with you when you said that it was the single best pokemon in the subway. I do agree that it is one of the most self-sufficient though. From my own experiences, I'd say that Suicune edges it out slightly (sorry for just copying what everyone else said hahaha), but honestly it is a close call because they can pretty much do the same thing anyway WITHOUT SUPPORT. This is why Dragonite is NOT the most self-sufficient because as the standard Dragon Claw/Dragon Dance/Substitute/Roost set kinda sucks without Whimsicott or some other crippler. It can set up on SOME stuff but not other things. It is still one of the best pokemon overall due to its versatility and Multiscale though, and it can fit on pretty much any team in some fashion anyway.
I guess I'll lean in favour of Suicune because it can set up on more things without assistance. I still have trouble seeing Scizor doing some of the things that Suicune did for me when I was using Zoroark (and this is WITHOUT Memento factored in). However, I'll admit that I've never used a Scizor with Roost (instant recovery is way better than Rest). Having used the Leftovers Scizor on my Sawk team, I can actually see where Roost would have come in handy at times, more so than Superpower (I used it like twice I think?). Maybe if I ever can be bothered to try it out I may change my mind.
Scizor also has the advantage of being way faster! Suicune is very slow since it's the kind of pokemon that HAS to boost as much as possible to succeed whereas Scizor can often make do with even just 1 Swords Dance (hell, I've swept with none before). Obviously it is best to get as many boosts as possible but I feel much safer with a Scizor with 1 Swords Dance boost than a Suicune with 2 Calm Minds (though on the other hand, I feel more comfortable with a Suicune with max. sp.att and sp.def than a Scizor with max. att.).
So yeah, based on my own experiences, I think Suicune is the most self-sufficient (again, I could end up changing my mind if I actually used Scizor with Substitute and Roost).
As for who is the absolute best pokemon (and this includes the dedicated cripplers like Whimsicott and Shuckle), I honestly don't know (I'm sorry for giving such a cop-out answer...). I've only play competitive Tower/Subway in 5th gen so my own experience is less than some of the other players here. I don't really feel comfortable saying "Dragonite/Suicune/Scizor/Luvdisc is the best!!!!!!" since I don't think I've played enough to know for sure (there are still tons of pokemon that I've never even used yet). I guess what I CAN so is that a pokemon is only as good as the team you put it on. I don't think there is a single set of 4 moves, nature/spread and items on a single pokemon that you can just team up with anything, though things like Bold 252hp/252def Suicune with Leftovers and Scald/Substitute/Rest/Calm Mind don't need 'as much' support as other things...
I guess that's where the question gets tricky since it depends on whether you are talking about a pokemon or a particular set of a particular pokemon. Meh...
Now that I have that out of the way...
I'm thinking of doing Multi again. I had success with dual Scarf leads before so I want to try that again. Not sure what I'll use and I may just mix-and-match to see what works best but it'll probably be something bland like Garchomp + Zapdos. I really want to get on the leaderboard!