Mosquitoes, hands down.
When I was around twelve years old, a mosquito bit me half way up my shin. Shortly thereafter, I felt a strong throbbing sensation in my leg, and the 'white part' of the bite was around the size of a golf ball. The 'red around it' went all the way down to my foot, and to above my knee; there was a rash that went all the way up to my stomach and the up sides of my body to my underarms. I thought I was going to die, and ended up having to go to the hospital. Needless to say, it was an absolutely horrific experience for me, and to put it lightly; mosquito bites are something I very much actively avoid.
Another annoying one in Australia is the huntsman spider. In summer, they find their way into my house quite often, and it doesn't really bother me, as their bite is completely harmless; I'll simply take them back outside. However, they're known to hide in and around your car; and that, my friends, is not a fun experience.
It was a few years ago now, and I was walking to the car just going about my business as usual. As I put my hand under the door handle, I felt something weird and sort of... 'fuzzy'? Before it even registered in my mind what it could be, a huntsman spider sped out from underneath the handle, and sprinted down the door to the underside of the car. Let me tell you, those things are
fast! I just about had a heart attack, and made damn sure my own car didn't have handles that they could hide underneath.
I've also heard stories of them hiding under the sun visors of people's cars, and creeping out at inopportune times whilst they're driving, or giving said person a similar shock to what I received when they actually try to use their visor.
Sneaky bastards they are.
I've had a few encounters with red-backs and whitetails inside my garage and on the exterior of my house, but nothing more noteworthy than simply seeing them... not that I'm complaining.
