1. Pegasi are the best type of pony - multiple studies have shown that they are the favorite among bronies, particularly for OC creation. Earth ponies don’t have magic or flying (lame), and Equestrian magic is too overpowered to make a balanced character. Having only one super ability (flight) is the best of both worlds: cooler than earth ponies, but not OP like unicorns or alicorns. Therefore, pegasus ponies are the best of the Equestrian ponies.
2. Rainbow Dash is the fastest and best flier in Equestria (See: Sonic Rainboom). So, she is the best at doing the one thing that only pegasi can do (and alicorns, but they don’t count, see point 1). Therefore, she is the best pegasus. Since pegasi are the best type of pony (see point 1), and she is the best pegasus, it logically follows that she is the best pony.
3. Rainbow Dash describes herself as "super-ultra-extreme-awesomazing," and since she is the best pony, we have to take her word as gospel truth. In addition, she frequently competes with other ponies (Applejack in particular), and routinely comes out on top. She is the best, says she is the best, and is always fighting to stay on top.
4. For the final nail in the coffin - I conducted a completely unbiased and scientific poll several months ago, and Rainbow Dash was voted best pony by a huge margin: 591 to 0. I'm not good at math, but I believe that's over 50%.
In conclusion, all evidence points to Rainbow Dash being the Best Pony™, with no other contenders even coming close to matching her all-encompassing awesomeness.
Leave your praise for our Lord and Savior Rainbow Dash in the comments below.
(Do you have your own irrefutable logic for who is best pony? Write a DA journal or tumblr post and link it in comments, I'll link it here for all to enjoy!)