Regarding annoying moment, sure, you can always find someone who will pick nits here and there, but that aside, if we talk about vocal moments that utterly blow me away, Freddie had countless moments on literally every record; Plant hardly has any, ever. Like I said, Plant is a good singer, but he has never been a singer that does stuff vocally that makes my jaw drop.
Honestly, the only rock/metal singers, off the top of my head, I would say can touch Freddie are Steve Walsh (although his prime years as a vocalist were shortened thanks to his smoking), Dennis DeYoung and Rob Halford.