I think the best rhyming word for songs (and poems) must be the word 'again'.

It can be pronounced (in southern England, at least) either as 'agenn' or 'agayn'; and the 'enn' and 'ayn' endings both include lots of useful, common words.

Added to which, the word 'again' is itself a common and versatile word, which is easy to include in a song.

Any other candidates for good rhyming words?