Ebay hands down. The absolutely last time I tried to log-in, I had to do a 'captcha', then get a secret code sent to my email, open the email, get the code, go back to Amazon, do another even worse captcha, then repeat the process because the code had expired. By the time I had finally logged in, I was so enraged I not only forgot what I was going to buy, I was quite ready to end Amazon. Then when I tried to close the account down, I was stopped from doing so by some 'catch 22', so I just disable the account by changing the email to a defunct one, and the password to a long one I typed out with eyes closed. Then I 'lost' the debit card Amazon had on file, so the bank had to issue another with a new number.
The most grief eBay has ever gave me was one 'captcha', and it was an easy one. Paypal on the other hand is starting to try to force me to get a cell phone, so I might be flipping off eBay in the future. Oh well, there is always Etsy and a host of other sources. Amazon and eBay think they are irreplaceable...they are NOT!