Here's my list.
6. Applejack - Didn't have enough songs to really hear, but the songs she did sing were pretty good, like at the gala. But other then that, no, not really a top-notch singer.
5. Rainbow Dash - Hate to put best pony on the list down like this, but she didn't have enough songs to stand out. The Junior Speeder's Chant technically wasn't a song, and Fluttershy did most of the pet song. But the Pet song WAS amazing, I have to tell you. If we had more songs with RD like that, she would be higher.
4. Twilight - Twilight is an amazing singer, the only problem is that she has few songs. Her voice for singing was spectacular, and brings out beauty, just like Rarity. Only problem is that she doesn't have enough songs, again.
3. Fluttershy - Fluttershy's songs are soothing, relaxing, and gentle. They bring out beauty in her voice, and in music. Who thought the most shyest pony would be a splendid singer?
2. Pinkie Pie - Pinkie has the most songs, and most of them are very catchy too. They are filled with fun, laughter, and everything friendship is about. Her singing is spectacular. The only problem is that her voice could be a bit more well-done.
1. Rarity - How do I describe this? Amazing. That's how. Her singing truly brings out the beauty in the world of music. Her songs are catchy, well-made, and the background ponies in the song have fantastic designs.