Sorry if my answer might seem a little long.
Firstly, I'll go through their pros, cons, and overall ability.
There are many great things about Greninja. First, his speed and jump height. He's easy to approach with and quick to combo F-airs and N-airs. His jump height is something to be reckoned with, even if you mis-click your recovery and use Shadow Sneak (as many players have done before), you will still be able to recover using Hydro Pump. Next, his combo potential. His throws have little to no knockback, and easy to link F-smashes or U-smashes into, if not a dash attack. His tilts fare the same way, usually leading into some type of smash. There aren't many viable kill setups you can do that are true combos, but you will be able to pull them off if you predict and think smart. Lastly, his kill potential. His meteor smash is bad, to say the least. It's incredibly hard to pull of on a skilled player, as it only activates during the first few frames. But-when used on the ground, or regularly, it can link into more D-airs or F-airs and B-airs, depending on which way they DI. Now, his most viable kill move is his U-smash, thanks to its two hit combo. Other good kill moves at high percents are his U-air, F-air and F-smash. Overall, Greninja is a hard character to master, but is very punishing when played with the right player.
Of course, the little electric mouse just may be the best of them all. He has a ton of recovery options, such as his U-special, S-special and wall jump. Against heavier characters, his U-tilt can be deadly, linking several of them into an N-air or U-air can give you an easy 40~%. F-smash will kill from about 90~%, and its range is awesome. His only meteor smash is the top of his U-special, but it's hard to pull of and leaves him open to punishes, so it's not recommended you do this often. Thunder Jolt is a good mindgame tool, combining it with B-reverses can throw your opponent off balance and keep them guessing. Thunder is a spam move, but it stales quickly. Link it from his U-throw or U-tilt at high percents for an easy kill.
Jigglypuff...well let's just say he's not the best. He's floaty, slow, easy to launch, and has mediocre strength. Nonetheless, he still has his perks. Sing+Rest is an easy kill at high percents, but only if you know how to use it. Pound is a good spam move, along with Rollout. His jumps, though, are the best thinig about him. He has 6 jumps, but no vertical special recovery. No meteor smash, but his aerials are among the best aerials in the game. Jigglypuff was known in Brawl for the best air fighter, and he's followed along the same path here in 4. Link multiple F-airs off the stage for a kill against heavy/slow characters. B-air is probably his best kill move, aside from Rest. U-air is good for juggling, and D-air is shield pressure. His D-smash has a crap ton of knockback, use it for disrespect purposes only.
Charizard is more damage than anything else. His S-special is probably the most commonly spammed move in his moveset. When used correctly, though (I mean U-throw + S-special) it can be destructive. Of course, it's always destructive, just...more.
His jab, his weakest move, still deals 7~10%. All of his moves have kill potential. All of this must come with a price, though. His recovery options are limited, though. He has three jumps, but his U-special is short, and his S-special sends you into a semi-fall state if you hit a wall.
Lucario is special, in the way that the more damage he has accumulated, the more damage/range/knockback his moves have. He has little combo potential, aside from his D-throw, at higher percents, so get damage early on linking F-airs and S-tilts. All of his moves, akin to Charizard, have superb kill potential. Aura Sphere is a great sniping tool and edge-gurading weapon. He's slow though, and his wall jump isn't that great. Overall, he's pretty easy to work with, and easy to start off with.
This announcement sent everyone into a wild frenzy. I don't think he's changed a lot from Melee. He's floaty, but not as much as Jigglypuff. His U-tilt has great combo potential, from both hitboxes. His F-air and D-air also have awesome kill-potential, and his N-air is good for combos. His N-special has a lot of kncokback to Mewtwo when used in the air, so much that some players link it to a spike after the initial knock back. His D-special and S-special are very, very gimmicky. They're more just things to play with than reliable kills, but they do have their uses. His D-special is devastating when landed, and his S-special can cause a lot of confusion (haha, never mind). His speed is average, and his U-special is his main recovery.
Now, who's the best? Well, I say greninja, but that's just my opinion. He has awesome combo potential, kill setups, speed, just the general feel of him. In reality, there is no best one. Every charcter can be played to become unstoppable (that's lookin' at you, Dr Mario). You can choose between the fastest, most powerful, most fun, most floaty, etc. Greninja is the fastest, Charizard is the strongest, but every character can be strategic. You just have to practice, and you'll be good at whichever character you wish!