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Some people are giving you perceived power levels rather than actual opinions based on experience involving each class. Pathfinder is dramatically different from 3E and class power is relative and based on what paths you choose.
1. Wizard: Still the most versatile and powerful class in the game at high level. It's impossible to match spell versatility. But still very boring at low levels and stands out as such when other classes are getting very cool abilities as they level up. No longer can do everything equally well to other classes. Like you can't make a wizard that does melee as well as a fighter or barbarian. You attempt this and you will get torn apart. But you still have incredible versatility with your spells at high level.
2. Cleric: Highly versatile. The most powerful healing caster in the game still. Powerful summoner. More general survivability than the wizard. Not far behind the wizard, but still can't match the wizard on sheer versatility and spell power.
3. Barbarian: You can build an absolutely sick beast barbarian now. A barbarian that can take most encounters and stomp them into garbage. A barbarian that casters hate because he only misses saves on a 1. A barbarian that eats magic, sunders spells, and can rip summoned creatures apart like cotton candy. That has insane hit points and can also dish insane damage. Probably the best melee class in the game right now.
4. Sorcerer: Because of a sorcerer's more limited spell selection, I place them on tier 2. They're still very powerful. What they lose in spell versatility, they make up for with bloodline arcana and bloodline powers. You can get all sorts of nifty abilities and make insane mind controllers with DCs that can allow you to dominate dragons. You can make blasters that wizard's can't touch. You can make shapechangers that can go toe to toe in melee. You can be immune to crits, stun, and combine powers to make for a beastly character that is very hard to deal with.
5. Druid: Druid is a druid. You get a lot of different abilities, but don't really stand out at anything in particular. You're a jack of all trades divine caster. You can gain access to some spells to make the druid a little more interesting, but it's still basically a druid.
6. Oracle: This class is pretty fun. Basically the divine version of the sorcerer. Oracle of Life is the best healer in the game and can be made really hard to bring down. Some of the other oracle paths are interesting, but not as interesting as the sorcerer. Some of the interesting oracle paths: Time, Metal, Dark Tapestry, Ancestor, Wind.
7. Fighter: Two-hander fighter is the sickest damage dealer in the game. Archer fighter is most dangerous damage dealer in the game. They beat you on initiative, chances are you won't live long enough to get a spell off. Archery does insane damage now. You can do a lot of things with fighters now. Even the two-weapon fighter is pretty nasty. I'm playing one right now and if you can feed on AoOs and get a good crit range, you crit a lot and do a lot of damage.
8. Monk: This class just got a huge boost with the release of the Ultimte Combat. Martial arts styles are fun and effective. Some of the new archetypes are cool and effective. You can make a sick Tertori Grappler or a non-lawful physically tough Martial Artists or a vampiric Hungry Ghost Monk or a Master of Many Styles using three or four martial arts styles together. Monk was already a pretty fun class, but they are more fun now. Probably still not as enjoyable if playing in a 15 point campaign, but if you get to roll or play in a 20 point campaign or above, you can have some fun.
9. Bard: The best physical damage dealer support class in the game. If you have a party with a few physical damage dealers, they will love you as a bard. Fairly good spell selection. They have one melee option bard now called the Dervish Dancer that looks pretty effective.
10. Magus: They did a real nice job on this class. Finally a fighter/mage hybrid that is appealing. They have some very intersting archetypes like Bladebound Magus and Kensai.
11. Rogue: Rogue is still the red-headed step child of the classes. If you like playing rogues, they can be effective. But are probably the least interesting of the classes. Bad archetypes and overall uninteresting abilities unless you play in a skill heavy campaign.
12. Ninja: This class seems very interesting. So far watching my friend play one shows they are more powerful than the rogue. Easier to set up sneak attack and far more interesting rogue powers. He's setting up this cheesy Sap Master build that any sneak attack class can do, but your DM will hate.
13. Ranger: This class is surprisingly powerful now. A ranger archer is vicious. Rangers have an effective spell list now. They are definitely a physical damage dealing class to be reckoned with. Really fun to play too.
14. Paladin: Paladin is a truly powerful destroyer of evil now and a near invincible holy warrior that is as hard to kill as any class in the game. Can have the best saves of any class in the game and gets more immunities now. Spell list is more effective. Smite Evil is a truly frightening ability now. When you smite an evil creature, they will feel it. Not once, but until they are dead or flee.
15. Inquisitor: This class offers a level of versatility that is hard to match. I can't really list everything they can do at an effective level. But so far I've been able to heal, fight in melee, and use a bow very effectively. I do so much damage with every hit I'm called the clean up man because once I bring the hammer, not much can withstand me. The amount of attack and damage bonus stacking you can do is insane. I'm averaging 60 plus points a hit at level 12 and I haven't even bought him optimal gear or had a chance to fully buff. Judgments are versatile and this class is very powerful and fun to play. I recommend melee over ranged if you play one, but I think a ranged guy could be real tough too.
16. Samurai: Ran one was an enemy. Does great damage to his target. Not a lot of versatility. I have to see more.
17. Cavalier: No experience with this class. No one has played one yet.
18. Witch: Can be nasty. Has some nasty hexes and a versatile spell list. Had one witch mess up the fighter badly with a hex. Not a great deal of experience with them. No one wants to play one as a PC because of the familiar. Somone kills your familiar, you lose all your spells. Very few people want to risk this.
19. Alchemist: No real experience. Friend says Ragechemist is a nasty archetype if you want to build a melee alchemist.
20. Summoner: Looks fun. No real experience with it. We fought a summoner. Eidolon was weak against optimized characters. AoE damage messed them up pretty good as the eidolon and summoner both took heavy damage.
21. Gunslinger: Too new. No real experience. Looks like it could be fun in a campaign with a DM that enjoyed running a campaign with an Old West feel.