I’m super-out of date, so talking about the originals here.
For the Decepticons, I have to go with Devastator / the Constructicons. Something about “We build for EviL” is just so cartoon villian-y that I’ve always loved them. Plus, neon green & purple.
For the Autobots - my first transformers ever were Gears and Cliffjumper - two relative no-name minis, but they have a special place in my heart.
Not a direct answer, but if you ever get a chance - go walk around a self-serve junkyard. This is where cars go when they finally just aren’t worth fixing anymore. It is eye opening. There are cars that you will still consider ‘new’ that have already given up the ghost (mainly Dodge/Chrysler, Hyundai/Kia, & Nissan). And you can’t help but think - WTF are these cars doing here, aren’t these still for sale at the dealership?
Conversely, there are also cars there so old you hardly recognize them (usually Honda, Toyota, and full size pickups from Ford/GM).