science

I like QuiteRSS (if you use Linux or BSD, it should be in your package manager). It's super-easy to sort and manage tonnes of feeds, it's very clear what is read and what's unread, and you can set it to never delete unread items. It hasn't been updated in ages, but it still works (mostly. Once in a while a Qt update breaks it until someone can fix the packaging).

My second favourite isAkregator. It's almost as easy to use as QuiteRSS, but unread items aren't as distinctive, and sometimes an unread item looks read.