When I first got into the show, I was a huge Fluttershy fanboy. I absolutely adored her, and I found her to be very relatable as well. I thought that she was always going to be my favorite pony, but as I got more and more into the show and its fandom, I started to become more and more interested in Rainbow Dash.
I didn't care much for Rainbow Dash at all when I first started watching the series, and I found her to be really insensitive at times. As the show progressed though, her good qualities became more and more noticeable, and I really loved seeing that positive character change. For example, in season 3, it's plain to see that she's become much more supportive and friendly toward Fluttershy, and she's not as conceited with her friends. I really loved seeing this change, because it's what the show is all about: ponies learning lessons in friendship and changing for the better as a result. It's true that this applies to all the members of the Mane 6, but it was so much more noticeable for Rainbow Dash to me, since I found her to be rather unpleasant in the very beginning. I ended up really liking her because of this, and she also became more beautiful physically to me. I just couldn't get enough of RD at that point, and she kept climbing and climbing until she overthrew Fluttershy as my favorite pony.
I'll always love Fluttershy, but Rainbow Dash really appealed to me over time, and it's interesting to look at things now and compare them to how things were back then. If there's anything I love, it's the feeling of something or someone you didn't care for initially to totally grow on you, and Rainbow Dash did just that.