The #1 class to play is Hunter. - easiest to learn, fastest to level, always highest or near highest dps (damage per second), even at max level. Hunters can get out of combat if they bite off more then they can chew. You get to have PETS. Awsome fun and you dont need friends to help you, when you got your PET.
Talk to anyone that has created a hunter, they will all tell you, with no doubt in their mind, it is the most fun class to play. Period.
2. Warrior is horrible to level up. - No healing, no way of getting away from Mobs. If you don't understand how to survive, you will give up on this class. There will be times, that you get poisen on you, and will die, simply because you can't get it off of you. This is annoying as hell, and slows up your leveling process. (pick this as your second character.)
this class takes serious skill to play and not die every ten minutes. It is seroius biz, behind a skilled player, but maybe give something else a shot first. WOW is a deserted below level 70. So you will be by yourself most of the time. this class sucks on its own.
3. Druid/rouge. Excellent class to play. very powerful, great survivability, Druids are fun because of shape shifting (Bear, cat, bird, seal) can heal, dps and tank. While leveling up, you can be any three of those things for a group, without being specc'd for any one specifically. Rogues have serious dps, great use in groups, and can get out of combat if they bite off more then they can chew.
Getting out of Combat is important, because you will want things to stop hitting you, so you can live. A few classes have no way of escaping easy, these two classes are very good at it, including hunters.
Both druids and Rogues can turn invisible (steath) which lets them roam around areas that have enimies that will kill visible players. this helps alot when questing, because you can avoid fights that would slow you down, or kill you.
4. Paladins. Excellent. This class levels very fast. is capable of tanking, healing and doing near best dps of all classes up to level 45. After that, they lose a bit of muscle, but can survive like few other classes. This is another fast leveling class. Almost impossible to kill a paladin, in normal fights. and when they bite off more then they can chew, they can run, jump, stun and flee to safety like no other class. not dying means faster leveling.
5. Priest / Shaman. Good. You can level this class very easy, As a shadow preist, you can still main heal for a group, while leveling up. this class has medium survivability. Shamans always make friends, because they are so helpful in a group. they can self resurect, so when biting off more then they can chew, you can rez without running back to your body, and they have two hearths. very nice indeed.
6. Warlock. Hardest class to play, nearly the most powerful class in the game. levels as fast a hunter, can self rez, and makes life a nightmare for anything in its path. don't get too excited, this class takes some skill, but worth the effort.
7. Mage. makes things go BOOM. you will either love this class or hate it. Fast leveling, cus things die.... like right now. but if anything hits you, you die. If hit n run is your style or you can think fast, then this class is awsome. Ice mage spec lets you get away when you bite off more then you can chew.
Warriors after level 65 can level pretty fast, because then their dps goes way up. but still a pain.
@ wowmom, a lot of decent advice. A lot of it wrong, or biased. Much of your advice is for level 80. a priest can be shadow and still main heal any five man, while leveling up. Priest is a skill class, and thus may not be the best to pick as your first class for solo leveling.
It does not matter what class has too many people playing it, almost everyone has multiple characters, so if he picks a rogue, he will still find a home.