I was once in your situation (I have 11 classes, 10 at lvl 90, 1 at lvl 89). I chose shaman as my 'main' and haven't regretted this decision so far.
First of all and most important, my criteria on choosing a class was that I HAVE to enjoy all 3 specs. Because I know that if there is at least one spec I loathed, I'd always doubt to progress this character.
Secondly, I had to decide what I enjoy the most between Damage Dealing, Tanking, or Healing. I decided that I love mostly DD and Healing.
Thirdly, I had to decide what I enjoy the most melee or ranged. From my experience of leveling all the classes, I found myself most comfortable with ranged classes.
These criteria have narrowed me down to choosing a shaman. Elemental is very fun and it's ranged. Shaman have a healing spec that I like too. And even though Enhancement is a melee spec, it's the only melee spec that I felt most comfortable with among other melee classes.
Again, your first step on deciding a main is to make sure you like all 3 specs on that class. If you can't find one, then it has to be at least 2.
I hope this will help you to decide your new main class