I will toss Symbian into the ring here.

I love it. Like their moto says "it is the mobile operating system"

-Symbian UIQ and Series 60 both do great multitasking given sufficient memory
-Symbian UIQ and Series 60 both have great native removable media support

Unfortunately, it is relatively nacent and is mostly devoted to games at this point.

Palm leads the field with 3rd party apps that made any device ideal for different users. I would be shocked to find 2 people that like the exact same device. For now, Palm gets my money.