Clockport USB boards would have to be number one. Thumb drives, external media, network, mouse, keyboard, printers, scanners, sound "cards" - and that's all on the humble Amiga!
Number two? Indivision - cheap SD/FFs? Lifesaver for many.
Number three? Hm. The ACA series of accelerator cards from Jens. I hear there's 2000 and 500 versions in the works.
Number four? MiniMig! And hopefully MiniMig AGA.