[Crazy Jetty]

I too would recommend the current TMNT line. It started fairly recently, and thus all figures are available and easy to get. No need to go to second hand to get older figures. And it's so widely available that there's no need to deal with scalpers.
Cheap too, averaging $9 usd for a decently articulated four and a half to five inch figure. Nice vehicle assortment, which are also cheap. And it has an absolutely MASSIVE sewer HQ playset that blows everything from the older lines out of the water.

Also good fo hit your transformer tastes would be Kreo Transformers.
The older sets got off to a rough start, but they're getting increasingly better, and the Microchanger Combiners are about the best toys under the transformer brand.

If you wanna deal with second hand markets, then Transformers Animated is a fairly short line that's full of awesome figures. Though it's a few years out of production.

Toylines like Transformers Classics (Which ranges over several lines: Classics, Movie 1, RotF-Reveal the Shield, Universe 1, Universe 2, TFCC/Botcon, Generations 1.0 and 2.0, Henkei, United), Marvel Legends, Marvel Universe, and DC Universe Classics though are so long running and so huge that they would be a massive undertaking and super expensive to collect.