1. storage space
If you a small business, then you won't need a vast amount of storage, if this is the case then the price will be small. Therefore, you will have covered both points 1 and 5 in one hit.
2. monthly transfer
This transfer bandwidth limit will need to be high if you have a lot of large images on your site, as everytime a visitor clicks on these it will put load on your bandwidth. Consider this if your site has a lot of visitors and there are lots of large files being clicked upon/downloaded.
The most important factor I would say bar-none. You don't want to be paying money for a site that can hardly ever be accessed as it's constantly off-line, whether it's a critical service or not.
Comes under up-time really and of course speed, which will all encompass a monthly transfer package that accommodates the demand users put on your bandwidth.
You get what you pay for, stay away from cheap packages that offer unlimited everything.
Hope that helps. As for most important, then uptime, but as you can see from my response ALL factors should be considered based upon your requirements.