I've used Dropbox, Google Drive, the MS offering Skydrive and iCloud. For me Dropbox is the best for accessing files.
I query why you are not willing to pay. The free Dropbox is good but if you have a number of documents and go above 2Gb, access whilst travelling is important, etc then I would have thought that it would be worthwhile paying something. You tend to get what you pay for.
Of course cloud access is only any good if you have a connection. You will need to make sure that wherever you are that you can get wifi, etc and reliably too. If using mobile data then you have to factor in the cost of that too.
I would have a portable hard drive too.