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Some carpenter bees will bite straight through the flower bypassing the stamens and stigma.

Sometimes it's just an anatomy mismatch - like very small bee species and big open flowers.

https://en.wikipedia.org/wiki/Nectar_robbing

Take a look at the Flowerpiercers: https://en.wikipedia.org/wiki/Flowerpiercer

They're true parasites, piercing the flower to drink nectar without any chance of pollination.

The most common example are ants. Moths are often guilty of this as well.

I'd guess hummingbirds.

Some plants actually have evolved to be pollinated by hummingbirds, they have long thin tube shaped flowers that a hummingbird beak can travel up. The Sword-billed hummingbird has an incredibly long beak due to mutual evolution with flowers that grew deep tubes.

No, hummingbirds also pollinate some plants. Random link from a Google search: https://www.nps.gov/articles/hummingbirds.htm#:~:text=Hummin...

Some is not all, and hummingbirds may well steal nectar from less tubular flowers.