[Afti]

Piloswine: High Tier
Availability: Late - to be precise, at the Giant Chasm. This is the biggest issue that Piloswine faces. It comes at a relatively high level, though, and it's easy to find; you don't need to baby it to get it up to speed with the rest of your party. Ideally, you replace an HM slave with Piloswine/Mamoswine for the last stretch of the game.
Stats: As Piloswine, it's a throughly mediocre Pokémon. But flying to the PWT, tutoring on AncientPower, and leveling it once gets you Mamoswine, which enjoys 130 base Attack, good bulk, and usable Speed. Mamoswine hits like a freight train; let's make no mistake about that.
Typing: Ground/Ice is fantastic. It's a pair of highly complimentary STABs, each of which is incredibly good on its own. Defensively, it's lackluster, but the incredible offensive power is well worth it.
Movepool: Small, but precise. It gets Earthquake soon after you obtain it, and can learn Ice Fang from the Move Tutor; combine those with Thrash, which it comes with, and Rock Slide or Stone Edge from a TM. The end result is a Pokémon that can level anything in its path.
Major Battles: At the time you get it, most of the game is done - but it's a typing that does very well against Achroma and holds its own against Plasma grunts. For the Elite Four, you can get several kills per match against everyone but Marshal; the Champion falls with spectacular ease at STAB Ice Fang and Earthquake. Common trainers aren't any trouble at all, either; even neutral hits will OHKO things with this guy's ludicrous Attack stat.
Additional Comments: Piloswine's greatest feature is easily how easy it is to use. It comes at a level near par with your team, and evolves with a single trip to the PWT; the ease of adding it to your party greatly cushions the availability issues. This is what lets it shine where things like Metang fail.

Are we tiering Dream Radar mons? I know we do the Pokéwalker, and there are some interesting options that open up if we tier Dream Radar - Riolu can come as early as your starter, at level 5, with a better ability; you can also get Sigilyph really early. Haven't even used Sigilyph in B2/W2, but starting the game with a level 5 Tinted Lens Sigilyph sounds like Magnemite-level god tier. Using Drifloon in my current run, too - shits all over early game pretty much entirely due to its Ghost typing, and mauls Burgh. Dwindles after Burgh until just after Elesa, when it evolves; haven't finished the run yet, but I'm expecting it'll dwindle late-game as well. At the very least, though, it's going to kick Marshal's ass.

I'm also going to hazard the guess that any of the Therian genies is pretty much unstoppable, ingame. Or Ho-Oh/Lugia/Dialga/Palkia/Giratina, for that matter. (level 5 Palkia is the best water starter)

if we tier DR mons, the list will probably go like:

DR ubers (prediction: Dialga, Palkia, Ho-Oh, Giratina, Lugia)
...
Therians
...
Magnemite
Zorua
Everything else

And, yes, you get DR mons early enough that they can effectively replace your starter - as soon as you beat Hugh for the first time, they're available. Exactly one level behind your starter; just walk into the Pokémon Center and withdraw them.