For me it was Dinosaurs, loved the latin names.
As a 6 year old I was given a couple of Transformers as gifts, and I loved the conversion gimick, but I preferred Lego (afterall the conversion there was practically anything I could imagine). I didn't become a Transformers fan until the Dinobots appeared in the cartoon, and what seemed like years later (at the time) on the toy shelves. There weren't any decent Lego Dinos at the time, I couldn't afford the decent Technic sets to make better ones, and the olny other Dinosaur toys were those tiny rubbery ones, and it'd still be a couple of years before Dino-Riders (the next series I really liked), plus several of the Transformers had psuedo Latin names.
I didn't become a serious colletor though until 1989 when my local newsagent started getting the Transformers UK and US comic books, and the characters got decent showcasing stories. IMO the only good stories were in the comics and Animated Movie, the TV series (apart from those episodes with the Dinobots) were just forgettable for me.
I've never been able to get into other Mecha series much.