4 RESWOB FO YELLAV
I went with Valley of Bowser 4, and went purely vanilla with the design (aside from the custom music).
For convenience purposes, my level is where Yoshi's Island 1 would normally be, and I put blue and yellow switches in the Yoshi's House level, since VoB4 has a few blue and yellow ! blocks in it.
(Also, I tested this with Snes9x 1.52 just to make sure none of the custom music I used broke.)
(EDIT: Fixed a typo in this post that I happened to notice, which would've bothered me if I didn't fix it.)