This should work for you:
RewriteCond /path/to/categories/$1.txt -f
RewriteRule /videos/the-best-(.*)$ /$1/videos/the-best-$1 [R,L]
It will parse the rule and look for the-best-(whatever), take that value and examine the file system for the file at the absolute path to your categories. Then it will do the rewrite. If you want to remove the category from the end of the rewritten URL:
RewriteCond /path/to/categories/$1.txt -f
RewriteRule /videos/the-best-(.*)$ /$1/videos/the-best [R,L]