"Main" is a tricky word for me when it comes to DDO. I find that it's very difficult for me to choose a main and stick with it. Every other day I want to play something else. I have more fun planning the builds of my characters and putting them to the test than actually playing through the game itself.
My longest played character is my mechanic rogue, but I would in no way consider her my main. I think I have the most fun playing my CC/Support Bard. But again, I wouldn't consider that bard to be my main. I think I can safely say I don't have one.
The closest character to a main for me would be the FIRST character I made: my Arcane Archer. I went through the quests and dungeons with that ranger for the first time and learned all about the game and the way it's played and how it really differs from P&P. It was the first character I made all my mistakes with. My ignorance made tons of people angry. Based on the nostalgia of going through the game for my first time, I would probably consider my arcane archer to be my main.
The reason why I chose AA was because I love ranged combat. Staying behind the group and picking enemies off one by one while the rest of my party goes in head first is what I enjoy most. Then when combat comes at me face to face, I love being able to switch to two weapon style. Primarily ranged and secondarily melee is my most IDEAL way to do combat in any game.