Hmm...
This is actually quite a tough one...
I'd have to say... that my greatest purchase... was... not technically
my purchase
Thanks to 1AZRAEL1 doing some great sourcing and procuring, he acquired for me my entire Beast Wars collection from the U.S and allowed me to pay him back, taking 2 years

, the $700AUD it cost to buy them and have them, plus a large number of Beast Machines and a couple of RiD figures that were thrown in for free, shipped over here.
The awesome thing? he didn't even tell me that he had done it!

He just showed up at my place one day with a box almost as big as him and said "Here you go".

...


The next 4 hours were spent exploring the treasures within...
As it turned out, it contained the entire Western release of Beast Wars figures save for a couple of Fox Kids Repaints and the Mutant figures that don't have a Bot mode.
It made up for what was missing though with a number of Japanese exclusives plus all the bonus figures previously mentioned and a pair of Convention Exclusive female RiD Decepticons (names escape me).

That's how you know a Best Mate, he had watched me for months struggling to acquire complete/near complete Beast Wars figures and knew that at the time I didn't have a lot of money and knew how much Beast Wars and Transformers in general meant to me.
It's probably not my greatest TF purchase really, but it was easily the greatest TF present I've received in my life, that's for sure!