Vergil, Wesker, Frank West (if just for the shopping cart assist) and Phoenix are Top Tier. Hawkeye and Storm are pretty rockin, too. Ryu, Dr. Doom, Deadpool, Cpt. America and Zero have all gotten some pretty hefty buffs, but not quite enough to make them Top. Plus, Doom has gotten his super best friend Strider back :3
Bottom Tiers: Shuma, Jill, Modok, Hsien-Ko, Arthur, V. Joe. Pretty much the same as the last game. The cool thing about MvC3's balance is that every character has SOMETHING that is OP in the right situation, but these just lack the same variety and tools that the better characters have. Phoenix Wrifght is, IMO, pretty bad. You basically have to design an entire team around him to use him effectively; its just far too gimmicky.
If you are new to the game I'd reccomend starting with simpler characters, like Ryu or Nova to start. They don't have many moves, and the ones they do have will seem familiar if you have any fighting game experience. All of the characters in the game have their own nuances, but they all play somewhat similarly (similar to MK, in that regard). Once you've learned the basics with one, they'll carry over to most of the characters in the cast. Just don't start with guys like Magneto, Dante, or Firebrand.
The most important thing to learn about MVC is that you are playing a TEAM, not 3 individual characters. This isn't KoF. Never design your team around the individual strengths of the characters. Instead, think of how they mesh. Some charactesr are terrible in a fight but have assists that can make another character go from good to great (Strider/Doom from MvC2, Wolverine/Akuma from MvC3). Phoenix Wright, Frank West, and Rocket Raccoon all have assists that completely validate their presence on a team, despite their lackluster presence in a fight. A good plan is to break your team down into a point man (the character you'll start the match as, and play for most of it), an assist (the guy who your going to be calling in to help the point), and an anchor (a "backup" character, to use when the pointman goes down).
Your pointman should be a good character, someone fast, strong, good combo potential. Many people choose Storm, Magneto, She-Hulk, Taskmaster, Dante, Vergil. Think of them as "If I could only choose 2 characters, who would I pick?" Your first choice should be your point.
An assist is much more specific, usually tailored to your point. The most important thing about them is, naturally, chosing the assist that meshes best with your point. As rushdown characters tend to be popular as points, many people choose assists that are good at applying pressure or hitting OTG to continue combos. Frank West, in particular, has a FANTASTIC assist in his shopping cart. Ditto Phoenix Wright with "Press the Witness".
Your anchor is like a second point character, with the caveat that they need to be someone who can basically win a match on their own. Someone who can come out strong, especially with the help of lots of meter and X-factor, and effectively "sweep" the other, weakened team. Sentinel, Wesker, and Phoenix used to be the most popular choices in 3. These are characters who may not be perfect on point (usually because they are very meter heavy), but are extremely dangerous in the right situation.
