I'm a big fan of TwiLuna. I've always enjoyed a love story involving two adorkable characters and in my opinion Twilight and Luna fit perfectly. Also in Luna Eclipsed Twilight was the first pony to approach Luna and spent the whole episode trying to help her.
I'm open to all ships (with the exception of incest and Twilight x Flash Sentry -just my opinion) as long as it is written in a believable way.