Wow, now that it's December 31st - looking back over the year and the sheer volume of figures I've acquired, this is actually a really hard question.
It's so hard I can't really decide on which one definitively, so I'll put down my top 5, ranked 1 to 5, if that's ok?

1. G1 Original Omega Supreme - He's complete, unbroken, still works and is a favourite because he was my 25th Birthday present from my parents, he's so iconic and this figure is as old as I am.

2. G1 Scorponok - My first Headmaster and bought from a best mate - his scale is just huge and as a Decepticon and a base, he is just so full of awesomesauce he easily makes this list.

3. G1 Micromaster Anti-Aircraft Base - This set has a lot of sentimental value to me as this set was a re-acquisition of the first Transformers I ever received in my life, courtesy of my late Grandfather, bringing back a lot of memories.

4. 2010 Reissue Predaking - The best way I can describe the feeling of what it was like to receive the Predacons and open them up is that it made me feel like I was a kid again, a feeling that hasn't surfaced since acquiring G1 Megatron a couple of years ago - This Gestalt, apart from being brand new and complete, is just 'wildly' cool in so many ways.

5. Masterpiece MP-05 Megatron - Aside from MP-04 Optimus and the Upcoming MP-09 Rodimus, nothing says 'Quintessential Example of Transforming Toy Perfection' as this figure - I had always wanted one after hearing about him and especially after seeing 1AZRAEL1's example, though I never thought I'd ever find one or afford one, you know you are a collector that has achieved something special when you manage to acquire this beauty.