Loads and loads of PowerCore Combiner dudes. They're cool, easy to transform and easy to replace. RTS Legend Starscream and Optimus! Possibly Universe Hound since I loved the guy.
A bunch of Battle Beasts guy! I've still got most of them I had in my childhood (from at least third grade forward >.> <.<) but I got more now.
One of my spare Sunspot, just 'cause I got a bunch of 'em
Tons of Legends or Cyberverse Legion to be in scale with the Metroplex I still have today!
I would also try to find a voyager class dude from the Cybertron line! I never had anything that side (aside form Metroplex but his transformation is boring!) back when I was a kid. I'd go crazy for Cybertron defense Scattershot or something! Heck even Mudflap!