For me:
1. just about any operatic aria (except "Summertime")

2. Pretty Woman (Roy Orbison) - I can't growl.

3. Most heavy metal.

4. Just about anything by Mariah Carey and other show-off uber-melismatic vocal stylists.

5. Anything below the tenor range.

6. And, if we're talking consistently and on-pitch & not making people's ears bleed, "Spanish Eyes." I have enough range of pitch, and I used to do the song pretty well, but in recent years I've had trouble finding a key where I can hit both the lowest and highest notes (and sound good), especially on the chorus where it goes up an octave and then down 1/2 step. DK why that's so hard to do well, but I find that it is.