Before I get to mine, since I took so long to reply to this thread, a few comments about some of the previous "capstones":
Patrick A. Riley -- I saw one of those Street Fighter Movie Ryu figures one time at a Convention. The collector wasn't selling it, he was just showing it. Weird looking thing. Had such a thick neck it looked like Ryu had swallowed a blowfish -- live. You're lucky to have one, though.
kingofpain26 - Congrats on still having your Buck Rogers' Twiki figure! I still have mine, too. He stands here on my computer table with a 9" Jem'Hadar and a bunch of little 1:144 scale NASCARs.
Cap -- NICE Stargate! What scale is that?
Taliesin Crow - That Night Creeper is an awesome item to have. Thanks for sharing it with us. Indeed, a special memory.
Python Viper - I've seen pictures of your Python Patrol customs and you have every reason to consider them your capstones!
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As for mine? Interesting question, and one I've never really given that much thought to. As diverse and extensive as my collection is, as others have, I cannot choose just one item. However, some serious contenders would certainly include:
The one surviving action figure from my childhood -- an 8" Mego Superman.
A figure that I very nearly forgot when my family and I moved one time -- a Space Academy "Loki" figure (technically made by Hasbro under their Aviva name in the late 1970's)
The OKTOBER GUARD -- and Kwinn -- just because of how incredibly overdue those guys were. And, I feel, they turned out darn well. Hasbro is to always be commended for these.
The U.S.S. FLAGG -- And Matthew, congrats on getting a nice table for it. Mine sits on the floor and I'm content with that, but it's interesting that for the average person, a table wouldn't be much of a gift -- unless you can put it to a good and specific use.
My G.I.Joe Convention Sets. I've never been disappointed in any of these.
My set of 9" Star Trek "Mirror Universe" figures from Playmates.