The question is deceptively complex. It really depends on how commons said creature is where you live, and what extraordinary attributes that brings with them. Anyone unimpressed with flies is obviously not thinking of Horse flies, or even gasp Blow flies. Being bitten so hard you bleed a river, or having maggots eating you alive are very much unpleasant thoughts. Be thankful if they are not native to your area or plentiful.
Much of the same is true with Spiders while the majority are fairly harmless to humans, and do tend towards shying away from confrontation. It is the exceptions that create disdain. Spider bites can be dangerous if the spider uses a potent venom, and what makes it worse is sometimes a person will not know they were bitten. This is a real concern where I live. There is a species of spider that uses a flesh eating venom, and people who receive a bite are rarely aware it happened. Until they find a patch of necrotic flesh. I just hate the though of rotting from the inside out.
For those that live in colder climates disease spread by Mosquitoes is hardly a issue, and to be honest the bites are not all that bothersome. Nature has too many things that simply apply a stronger itch. Poison Ivy, Oak, and Sumac can really lay on the hurt. Corn and Nettles even add a touch of burn to their itch. As far as blood suckers actually go Fleas are harder to control, and Ticks are truly the disease ridden gluttons. They can get as fat as your thumb, and unlike a Mosquito that dines and ditches. A tick will literally burrow into your flesh. Oh the memories I have of my mother giving me a head check for those vermin, and having to dig them out with tweezers, and a cigarette lighter.
My two cents it has to be spiders. Most everything else is easy enough to deal with. You can wipe flies out with a saucer of milk and toadstools. Mosquitoes can be put off by a little smoke, and if you do get infected by a disease there are treatments for the more common ones. Cockroaches are rarely a problem for those that do not decide to live in squalor. No a spider can kill all on its own. So those are the ones that most concern me.
They are all of those things to me, because they can kill me. However I do not despise them. I just have a very healthy respect for what they can do to me. I do not hate living creatures for doing what nature intends for them to do. I am the smart one it is up to me to manage their impact upon me. Well except for the common house fly which is a domesticated species. Yes it is wholly dependent on humans for its livelihood. In that case it is our own damn fault. We created a dedicated garbage disposal to clean up after ourselves. I am not sure we are justified in complaining about them. Even if we hardly use them for that anymore.
Perhaps in a little twist of irony our modern annoyance is actually being oblivious to the fact that the species in our distant past was supremely beneficial. They after all probably kept our dwellings clean, and in doing just that they may have cut down on the occurrence of both disease and predation. The existence of the species is likely a telling sign that our ancestors were hardly fastidious when it came to cleaning up after themselves.
