Interesting discussion.
Best return was my Stanley #5 and a starret combination square. Those two tools literally touch 95%of the work I do if not more.

Worst return on investment is even harder. I hate to be "that guy" but I'd have to say the Lie Nielsen #8 that I bought new. I thought it would be a game changer, but I realized my roundside #7 flattened by tablesawtom was just as good for a third the price. Hard lesson learned.