World of Warcraft is not the clear-cut best choice.
Character progression is very solo friendly but the 'End Game' is the opposite. There will be almost nothing for you to do at Level 60. PvE revolves around 5-40 person content; and PvP revolves around medium to large scale battles. Unless you're a Rogue with stealth, there's little hope for solid solo world PvP either.
I'm not sure there is any other solid solo End Game available right now though. The most solo-able End Game of recent MMOs was Star Wars Galaxies, but from what I've heard they made the game much more group-reliant. Any other MMO that may offer a solo End Game to you is probably too old to suit your tastes.
So while World of Warcraft may still be your best bet, just remember that once you reach Level 60, there will be nothing left for you to do as a solo PvE player. If you hate that idea, you may want to do some research on Star Wars Galaxies, City of Heroes, or perhaps even Asheron's Call if you don't mind retro 3D graphics.
As an extra point, if you do enjoy World of Warcraft Level 1-60, you can always make a character in the other faction (Horde vs Alliance faction war) or take a break and wait for The Burning Crusade expansion which will increase the level cap to Level 70 and add 2 new races (and new Level 1-30 content) which should give a casual gamer many more months of solo gameplay.