As others have said, this is largely a matter of opinion. That said, here are my opinions!
Defender: Fighter is still the king. Stickiness, decent multi-marking capability (highly build dependent, though), and easily capable of Striker-like damage all combine to make the Weaponmaster Fighter the best Defender out there, the one by which all other Defenders are measured. It's a pretty robust field, though.
Runner-Ups: Knights break down in Paragon and higher. Shielding Swormdages are a unique take on the Defender role, but Assault and Ensnaring Swordmages are strictly sub-par Defenders. Paladins are capable, buoyed by some truly ridiculous PHB1 Paragon Paths. Wardens are all about survivability, but are capable. Battleminds had some big issues. They've received some fixes since their debut, but are still suffering from want of a strong MBA. It's frankly silly that a melee class, let alone a Defender, doesn't have easy access to an MBA.
Striker: If it's Martial, you're good to go, baby. Rangers are top dog, though, and likely will be forever until Twin Strike is nerfed. Martial guys typically have it better than implement users, however. They have access to the improved Expertise feats, superior weapons, multi-attacks, minor action attacks, immediate reaction and immediate interrupt attacks, better support in the form of Martial Power
and Martial Power 2... everything you need to get your damage score higher.
Runner-Ups: Basically, everybody else. Barbarians fare decently, provided you go Whirling Slayer or otherwise sacrifice your secondary stat in favor of Dex. Pump anything but Strength and Dex, and you'll find your AC is hilariously low and you become the true definition of a glass cannon. Sorcerers can make due by playing to their strengths; find all the static mods to your damage you can get your grubby little mitts on, and the focus on bursts, blasts, and multi-attacks to make the most out of it. Few Strikers are out-and-out bad, with the possible exception of the old-style Assassin. Executioner Assassin is passable, I've seen them in play. They pair well with Warlords.
Warlocks... eh. I want to like them, but I just really don't buy the whole 'single-target controller' angle. I think a Sorcerer-King Pact Warlock can make due in Heroic at least, thanks to that one feat that gives them an extra 1d8 psychic damage or whatever. They'll probably have to poach powers from the other flavors of Warlock, however.
Leader: Warlords, hands down. If your party isn't the right party for a Warlod, wait for the inevitable TPK, and then remake as the right party for a warlord.
Runner-Ups: Cleric received some serious nerfs lately. They're now strictly the "best healers," but so what? All healing does is stretch out a fight. Warlords win for enabling their allies to take extra attacks. If a Warlord really and truly doesn't work for your party, then Shaman or Bard do just fine.
Runepriests and Artificers both suffer from a lack of support, but Artificers have a little bit more going for them due to a few really good powers.
Controller: Wizards, due to the sheer number of options they have and the great support. Invokers get honorable mention, since they get access to some truly amazing (save ends) effects. If you can capitalize on those, you're golden. Psions are good due to the spammability of their early At-Wills.
Runner-Ups: Binder and Seeker are messes, and more or less regarded as total failures. Druids are pretty good, but need to be built carefully. You can easily construct a glass cannon melee guy or a Wizard with crumbier powers.