Well, I've been going through my annual "what's the best pdf software" struggle. So here's my latest update:
For the first time, I have to agree that out of the options, my favorite viewer is PDF-Xchange. It's fast, it looks good, and it has tabs (you have to have tabs!). Very good program. Currently, my preferred viewer.
Nuance is also good, but definitely not in the "fast" category as far as starting up. Once in the application, it behaves quickly. To me, speed is two different issues: startup speed, and operating speed. Nuance is great for operating speed. Also, nuance has nice layout features and powerful editing features. But I wouldn't use it for my viewer.
Bluebeam Revu. This is my preferred editor. When I need to really work with pdfs, this is what I use. it can do anything and does pdf markups/creation/editing better than any other program available. Again, like Nuance, I wouldn't use this for my viewer because it's too slow for that. But it's not meant to be a viewer only.
Sumatra is fast and all. A little too simple for me. I prefer PDF-Xchange over it. I do use Sumatra on my portable drive, however.
Foxit. It's okay. Not my favorite in any category, though. I prefer PDF-Xchange over it.
So, in summary:
portable viewer: Sumatra