I think listing a company that was formed 5 years after Commodore went under as a software publisher that changed Amiga History is more then a little strange.
I must admit I was thinking of the potential of AOS4 and attributing that to Hyperion. I chose along the lines of "keeping the Titanic afloat" type efforts. Without them, the Amiga would be history. Otherwise I agree with several others listed.
My vote for the top five:
1. Newtek - probably had the largest impact
2. Electronic Arts - the IFF standard and, for me, F/A 18 had me hooked the minute I saw it.
3. Psygnosis - Many people over the years who knew nothing more about the Amiga knew about Lemmings
4. SAS Institute - The long-time standard C compiler
5. Ralph Schmidt along with Phase5 - made the first move to PowerPC
A few others worth mentioning:
Aminet/Urban Müller.
Fred Fish
William Hawes
You can tell the newbies by there lists.
P.S. I bought my first Amiga in 1987, and my first PC in 2001. ;-)