Well, the thread was started in 2015, so my best purchase of 2015 was my 19" x 80" LeBlond engine lathe.
If you move on to 2016, my best purchase was my Mitsubishi forklift.
If you include non-purchased, but still acquired, my late paternal grandfather left me his toolbox full of treasures. My living maternal grandfather gave me a wilton 2" baby bullet we found in his barn. 'Twas a good year in tools, especially ones with some heirloom significance.