I agree with Adamantx's assessment of Flareon, though Fire as a defensive type isn't the worst in the world. I think C-rank behooves him.
However, he might just have some competition...
Rampardos GO!
Rank: C
Rampardos has always been my favourite Fossil mon, a position that has only been reinforced now that second-place Omastar has been hijacked by Twitch. However, Big R's not without problems. He's slow and fragile, particularly on the special side. However, with proper support, I think he has a place as a lategame missile, not least because of his obscenely high Attack stat. Here's the set I use.
Rampardos SMASH!
EVs: 4 HP, 252 Attack, 252 Speed
Ability: Sheer Force
Nature: Adamant
Item: Choice Band
Moveset:-
--> Head Smash
--> Earthquake
--> Superpower
--> Fire Punch
It's a fairly standard set from Gen V, but then again, Rampardos didn't get much in the way of new goodies upon arriving in Gen VI. Besides, Rampardos has always been a simple Pokemon with a simple job: Hulk out lategame with the most powerful Head Smash going. Oh, how I long for the day when Mega Rampardos shows up with increased bulk and Rock Head... but we're getting ahead of ourselves. The point of Rampardos is to kill stuff with Head Smash. That's about it. This is for the quite understandable reason that a Head Smash from Rampardos is very bad for your health. Head Smash OHKOs Delphox, Froslass, Hitmonlee, Sharpedo, Aromatisse, Zoroark, Slowking, Yanmega (hoo boy, does it do nasty things to Yanmega), and sundry others. It can dish out some serious hurt to Rhyperior with Superpower, OHKO Durant and Escavalier with Fire Punch, and do ghastly things to Cresselia and Reuniclus with our phrase of the day. Moltres looks at a Rampardos Head Smash with the same expression a fretful child has when locked in a room full of clowns. Cobalion can be eviscerated by Superpower (which I'd use to dodge that nasty Magnet Rise set doing the rounds at the moment), Druddigon gets OHKO'd with Head Smash, Heliolisk and our new friend Jolteon get blown apart by Fire Punch. Gligar is dead and buried if you can find enough left to bury. Registeel can just barely avoid being OHKOd by Superpower if you don't have Rocks up. Gallade dies. Virizion dies. EVERYTHING DIES. ALL IS ASHES AND SKULL FRAGMENTS. RAMPARDOS IS THE KING OF A DEAD LAND OF SHATTERED MONS!
So why am I only giving it a C rank?
Because half the time, you won't get to kill anything. Rampardos is slower than an arthritic tractor and dies if you sneeze on it. Sure, it has the most powerful Head Smash in the game, but that often works against it; since it has bugger-all recovery and lacks the one ability it really needs to succeed (GAMEFREAK WHYYYYYYY), so the recoil from Head Smash often causes the poor dino to keel over. That's why I sometimes use him as a lead; other people have hazard setters, and they may well be able to get a hazard off, but you have spinners and Defoggers for that. Obliterating them with a move of your choice is the best way. In short, with the right support, Rampardos can batter aside pretty much everything in the tier. He's a brutal wallbreaker that can rip apart enemy defences, but make no mistake, he will die doing it.
Hopefully, this sparks some discussion. =]