Oh, good question! But I find it's so difficult to make a SINGLE CHOICE. Bcz... em, many dialects are beautiful :)
In my impression, Wunese (not Strictly, including many different but generally similar dialects in Jiang Nan area)sounds so beautiful. Many poets and dramatists lived in Jiang Nan area. The poems and drama they wrote sounds beautiful when you read it in Wunese.
My hometown, Hubei province, in ancient China named Chu (including both Hubei and Hunan). The dialect is also melody (allowed me using this word to describe^_^). Qu Yuan, a very very important poet in ancient China, lived in Chu. If you hear his classical work Li Sao (离骚. Li,离=live; Sao,骚=sorrow) read by our dialect you'll find incredible fascination. As the work shows.
Mandarin is also very dulcet. It sounds clearly with power. Sometimes I thought it's so great to read heroic poetry, modern poetry and novels with Mandarin.