I personally love dieresis and cedilla...
What's your favorite French letter with diactric?
The circumflex, especially over the i and o is my favorite (nice and symmetrical, and not as fussy/messy/trifling as the umlaut/diaeresis).
There are only 15 words/ names with ' ÿ ' according to wikipedia and my knowledge... As for circumflex I love it above 'i' and 'e' - but that new spelling proposal decided to remove that accent from 'i' and 'u' :(