I remember Yahoo captcha's to be the worst (maybe because I ran into them most), for being actually unsolvable, but those were the messed up letter style of captchas days, which all had seemed to have that struggle.

If you want to go much deeper with finger printing, this page has links to a whole list of different specific kinds of fingerprinting online checker sites along with a good brief summary of some. I'd call it a good 2nd entry-level quick dip into the topic:

https://browserleaks.com