Ha ha, great thread actually. But where does one start? My initial reaction was to start categorizing inventions/discoveries into different scientific fields. In the spirit of all other fence-sitters out there I'm not going to commit myself to one answer... yet. Here are my top five discoveries (either thrilling or important, in no particular order)
1. The Big Bang - Discovered by Penzias & Wilson and expanded upon by many others, but the fact that they initiated the discovery that we live in an expanding Universe with a microwave receiver is pretty damn thrilling
2. The Germ Theory of Disease - Go ahead, count how many people have been saved by this discovery? Sorry, you'll never have enough hands. In competition with number 3 on the list for most important medical discovery
3. Immunisation - Undoubtedly one of the single-most important medical discoveries in history. (I expect to see X-rays, anaesthetic and antibiotics on this thread as well)
4. Iron Processing - Pretty darn huge discovery, though it happend about 4000 years ago.
5. The Steam Engine - Don't knock it yet! From mills to factories to trains to boats, it catalysed the Industrial Revolution in behemoth scope and literally influenced the entire way our world now works.
I so wanted to put the disccvery of the genome and DNA in there but resisted since there have not been any direct, broadly applied benefits from the discovey (and since it's medical I think that should apply)