I went for priest, mage and three warlocks.

That being said, the reasons were not only the benefit these classes have from each others abilities and the fact that I wanted a warlock on my main account and a priest on a secondary account.

I think the most critical thing for your enjoyment is how feasible your class combination is. Not just in game terms of how the classes work with each other and can fill the the dps, heal and tank functions, but how the classes are played in game. Just because a two classes work well with each other when played by two seperate people does not mean that this will also be the case if played by one person. Certainly, that is part of the challenge, but you will also want things to work fairly smooth to begin with.

My advice is therefore to:
- think about how the classes that interest you benefit from each other,
- make sure dps, tank and healing is covered and
- consider how the classes are played in game with respect to you doing both at the same time.