Raboot, Buizel, and Piplup! I absolutely adore all of these pokemon and while I did eventually evolve them, I delayed it for quite some time so I could enjoy them more!
I was the same for Buizel in my last playthrough of Platinum, I absolutely love that Pokemon so decided to keep it around for a few more levels! I sometimes do that with a few other Pokemon as well, but generally, I'll stop others evolving just so they can learn some new moves.