Well, if you're looking show-wise, I would recommend finishing out Prime. You could try the new RID show, but it's less serious and more kid-oriented than Prime.
Other than that I don't know because I am also a Transformers casual.
Although I know that you probably shouldn't do anything with the Unicron Trilogy (Armada, Energon, Cybertron). I've heard that Armada and Cybertron are decent, but Energon is crap.