1. "What is the best race/class?"
The races and classes in the game are well balanced. If you make a post asking "What is the best race?" or "What is the best class?". be prepared for flames. There is no uber-class/race. Visit the class forums and read the stickies there. Do your homework instead of asking others to do it for you. In the end, class/race is an individual choice. Pick one you think you will enjoy playing and try it out. In fact, pick several and try them all out. You should also look over Ideggeries Post
Beginner's Info.
2. "What is the best class for PvP?"
After looking over many, many threads on the subject, the consensus seems to center around rogues as the best PvP class. Rogues have many advantages. Stealth, stun-lock, and high DPS seem to be the reasons that rogues are favored. Warlocks come in a close second.
3. "What is the best class for leveling?"
After looking over many, many threads on the subject, the consensus seems to center around Hunter as being the best class for fast levelling. High DPS, coupled with a pet that can tank seem to be the main reasons behind this.
4. "What are the best classes for duoing?"
Almost any two classes can get together and duo succesfully. By spreading damage across 2 people, there is less downtime. Almost every combination has been put forth as "best", but it's really just another choice. Choose classes that both of you are comfortable with. However, you really need to choose your race carefully. If you choose Alliance, you can have any mix of Dwarf, Human, or Gnome, or choose two Night Elves. The reason for this is that getting an elf to the other alliance starting zones is dangerous and difficult. If you decide to play on the Horde side, any mix of Orc, Troll and Undead is fine, but again, if one person chooses Tauren, so should the other.