stock in Walmart or Microsoft is the first things that comes to mind...lol
let's see, you got me to thinking about this one Jim. (you always have such good questions)
I thought maybe my metal lathe, but it really came along at about the right time. my little Dewalt table saw with a really good blade is something that I think I wish I'd bought long time ago. of course there is a host of steam engines that I wish I had Bought a long time ago when they were relatively inexpensive and plentiful.a good set of micro Precision screwdrivers.
I don't have too many regrets, when I need a better tool I buy it, and am delighted with it.