I'd be interested to hear what you like about them as well, but it's up to you. My personal favorites (in random order) are;
"True, True Friend" (G4) - I find this song to be really uplifting, and it just seems very fitting for the show's theme; it is an excellent song from an excellent episode (in my opinion) and I am often singing it to myself, it just sticks with me.
"Shine On" (G3) - This song is, to me, very heart-warming. I really care about the meaning of this song (and the episode) that you shouldn't be afraid of being who you are, or what others will think of you. The harmonizing of the characters are in perfect synch with each other, and this beautiful song always brings tears to my eyes when I hear it (I've even got some tears now, and that was just from thinking about the song...
)
"Sea Ponies Song" (G1)" - G1 songs are actually pretty low on my list for the most part, but I love this one a lot. It's just a nice happy song, and fully of energy, it really sounds like something I would have enjoyed as a kid, so it has a nice nostalgic attachment to it. If there had been more G1 songs like it, there would be more G1 songs on this list.
"Cutie Mark Crusader's Theme" (G4)" - The song from the episode that brought me (back) into the fandom. It has a nostalgic and sentimental attachment because of that fact, but I can also relate to the lyrics. Even though the song is purposely out of tune because of the storyline (the CMC all did the wrong thing, the only thing done by the right pony was the lyrics) I can see past that, and it made know that I would care about the CMC, and that I had to be a fan (though they are now only one fraction of what I love about the show, I still care about them a lot). I also love how they built it up during the episode, and then shown in full at the end.
"Make a New Friend Everyday" (G3) - I'm not really sure what I like about this one, but I know I love it. It has a nice catchy sound to it, and the simplistic lyrics are actually one of it's strengths to it, I guess it has a nice charm to it (it's actually my second favorite G3 songs, the first being "Shine On").
"G3.5 Themesong" (err...G3.5) - By far my favorite theme for My Little Pony, ever. It makes me feel really uplifted when I hear it, and strangely nostalgic, even though I didn't discover it until after I became a fan of G4, it feels like I'd been a fan for a lot longer. Ironically, it was via a G3 hate video that first introduced me to it, although it was actually a lot tamer than most hateful material (yes, I know it's G3.5, not G3, but since when could the haters tell the difference?) and may have been the stepping stone, to me remembering that I had loved My Little Pony before G4 (since seeing it reminded me that there were shows before MLP: FIM).
"Art of the Dress" (G4) - Not only does it sound really good (and a good tribute to the original) it really helps me relate to Rarity in this episode, I can totally emphasize with her here, so it's nice she got a good sounding song for it.
"Positively Pink" (G3) - This song shares some factors of my views on "True, True Friend" (that I can't stop singing it to myself) and "Make a New Friend Everyday" (that I love it a lot, but don't know why exactly).
"Smile" (G4) - Not only does it do what it sets out to do, it has a nice nostalgic attachment to it, because it sounds like something that could have been released at the beginning. I love how it practically has its own music video, and brings me back to the time when it probably would have been released as a real song. I also love how the FIM team teased us with it by leaking it, and not saying when exactly it was going to be in the show. By the time it actually did make its appearance, many bronies that wanted to hear it feared that it was either never going to be used, or it would be too forced and not really work after all the hype, but in my mind, it did.
"Love's in Bloom" (G4) - It has a lot of things I like; is romantic, has a nice 80s sound to it, and sounds like something that could genuinely be played. It was by far the best song of the episode (I think I'm the only brony in existence who didn't like "This Day Aria").
As a side note, I also love how the My Little Pony franchise has kept the same opening bars in almost every song it did, both in the shows and a couple of commercials too.
What I mean is, the one that starts with ten notes (usually the lyrics here are;
"My Little Pony, My Little Pony..." then six, seven, eight or so notes for the next line. Tales didn't use it, and it had no lyrics in My Little Pony and Friends (it was the instrumental part of the song).
Hope you didn't mind the long post there.