I think the current correct content types are:
'application/rss+xml' (for RSS)
'application/atom+xml' (for Atom)
'application/rss+xml' (for RSS)
'application/atom+xml' (for Atom)
As soon as a majority of sites use the correct types, clients can start requiring it for newly added feeds, which in turn will make webmasters make it right if they want their feed to work.
'application/rss+xml' seems to be the best option though in my opinion. The '+xml' in the media type tells (good) parsers to fall back to using an XML parser if they don't understand the 'rss' part, but the 'rss' part provides more accurate information on the content's type for parsers that do understand RSS.
All that said, it's a mess.