[Snowflakian]

I don't think I could pick just a single figure as the best ever yet, (Hasbro get cracking on an X-23 or Drift figure and I might change my mind on that) but Unicron, Primus, and Fortress Maximus do rank up there.

I'd have to say for now though, the mighty lego brick is my top choice. Compatible to all toy lines, and with enough bricks, you could even build stark tower in scale. The only limit to them is your imagination.

That or Animated Shockwave or blitzwing. They are like having several figure in one with high articulation.

Energon SnowCat also ranks up there for being the first land joe vehicle done as a transformer.