I think if I were planning to age beers I'd get a few of them by the case, so that you could compare as the years went by, and have plenty on hand when they hit their peak. Obviously a certain amount of self-restraint would be needed!
To answer the original post, I'd get some Samichlaus and Chimay (blue), and Westvleteren if possible (this is where the trip to Belgium would come in <!-- s:D --><img src="{SMILIES_PATH}/icon_biggrin.gif" alt="

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I see that the vintage beers article linked above lists Sam Adam's Triple Bock as a recommended beer for ageing - I have a bottle of this at home but have never been tempted to open it as I hear it's complete muck - anyone tried it?