Huh.... excellent topic: what was your gateway TF into collecting as an adult?
I'm thinking real hard.... I think MP11SS (Sunstorm) is what sucked me back into the MP ring in 2012, MP10B (Black Convoy) in 2013, the Henkei Seekers in 2008, the 2010 Unicron and Primus, and the TFTM characters released in the Henkei/United line (2008/2010). But I know I was very late to the Henkei game, but buying United figures shortly after their release.
SO:
-2010/2011, United/Henkei figures that started my hardcore Generations collection, likely instigated by Primus/Unicron.
-2012/2013, after ignoring the MP line for a while I aquired Sunstorm (being my first of the "new" MP Seeker mold) and Blacky Convoy (being my first of the "new" MP Optimus mold). After both of those, I was like "holy, they're very different from the OG's!" and decided to go to town