The YI Castle theme will ALWAYS be #1 on my list. Here's a few songs that I saw that I like as well.
I also think that the YS World 6 theme is great. I mean, I remember the very first runthrough of the game... I mean, I had only 1 Yoshi left (And, in hindsight, I realise just how bad you have to SUCK to only have 1 Yoshi when you reach W. 6) and the music made me play far more cautious than I would have (One of the few times where a graphical/musical influence has affected my gameplay...) ... but I still won.
Anyways, Aside from the 5 I originally listed, I also like:
The MK: SC Rainbow Road song (Especially the first few measures)
And the General Guy battle. How'd I forget about that one?
I know there's more...
Everyone has their own opinions, no matter how WRONG they are.