Oh, gosh. There are so many great ones.
Starting with the ones I listen to the most, most of which are from Equestria Girls:
Let it Rain
Find The Magic
My Past is Not Today [particularly the slowed/daycore version]
Discover the Difference (it was a game, but I love the song)
Then there's also a bunch of other Equestria Girls songs... Though I could probably point to any EG song and say that I love it, these are my top:
True Original
The Legend You Were Meant to Be
Legend of Everfree
So Much More to Me
Life is a Runway
Dance Magic
Run to Break Free
And a handful on non-EG songs:
The Magic Inside
The Pony I Want to Be
Celestia's Ballad
Luna's Future
We're not Flawless
G3's Shine On (the special and the song are garbage, but I love them. I want to redo this song some day)
Probably the Friendship Gardens main theme because of nostalgia
Then I'm like... constantly finding myself singing or humming these:
Let it Rain
Find the Magic
Welcome to the Show (I can sing just about this entire song, I often end up just going to it by default whenever I feel like singing)
Fluttershy's and Sweetie Belle's Lullaby
Part of the CMC theme song, funny enough, another one I think it would be fun to make a cover of
Then there are some really good remixes and fan-made songs... I can't not mention a couple of them.
Daniel Ingram - BBBFF (Regret is Forever Remix)
EM120X - Fluttershy model music
InfinityDash - Fluttershy Model Mix
JayB ~ Giggly Maria - Rain (Original Mix)
As for why, well... Most FiM, especially EG songs just happen to be in a style I enjoy listening to, and I appreciate that the emotions in the songs are often both fairly charged and direct. They're really great for relating to, and for being inspired by.
I'll share a few of these mentioned as videos: