Traditionally, ceylon is the tea of choice for flavored tea. I believe the reason for this is because ceylon is very delicate, so it can be successfully flavored with ease.
However, I have found that I really enjoy flavored assam teas. I like the fact that I can still very clearly taste the tea under the flavoring. As for what flavors to use, the only ones I have used on assam thus far have been citrus (such as bergamot and lemon).
I also like to add vanilla to many teas (I haven't tried it in assam yet, unfortunately). Vanilla.com sells some very high-quality vanilla extracts which you can use to flavor teas.
If you are looking at flavoring teas, I would recommend picking up samplers of the various teas you are considering. Make a cup, and try to think of the foods you would like to have with this tea. If you have trouble imagining flavors, try the food while drinking the tea, and see how it pairs. I don't think it will take you long to get the hang of it.
The most important thing to keep in mind is to pair stronger flavors with stronger teas. If you pair a strong flavor (such as lemon) to a more delicate tea (such as white tea, or chamomile), you will only taste lemon flavor. What a waste.
If, however, you were to pair a stronger flavor with a stronger tea (such as lemon and assam), you will be able to taste both the tea and the flavor you added, resulting in a tantalizing ensemble of flavors and aromas.
Sorry I can't be of more help. I will let you know if I come across any shockingly yummy flavor combinations.