I find this really difficult, because the ones I like most are usually the ones I sing myself - which I sometimes don't like at all before I start singing them if that makes any sense ...
I am not a big Mozart fan in general, but some of the arias I love most are the ones I performed myself when I was still mainly singing classical. I performed Sesto and Cherubino several times, that's why "Deh per questo istante solo", "Parto, parto" and the two Cherubino arias are the ones I still love the most (Sesto more than Cherubino though). Visually, I was always very much the pants role Mezzo anyway, vocally maybe just when I was in my mid-20s
"Soave sia il vento" from Cosi is also a favourite since I have done Dorabella in a students' production (unless you have a bass-baritone who lets you and the soprano die by taking time in his cantilena). I am not too keen on the other Dorabella arias though (mind you, "Smanie implacabili" is very good to let your inner diva hang out

). Like Mozart in general, they don't sit particularly well in my voice. When I was still younger and the voice a bit lighter, I sort of got by (although I already found it quite hard work), but the more my voice matures, the less I like singing Mozart myself. Still, like Rossini, an incredible training tool though.
I absolutely love the Dalila arias (all three), but I fear I have taken a different path, so I won't sing them anywhere apart from in my own four walls
I also love "Carmen", but mainly the Card Scene ...
Tant pis! (nah, not really

)