If you want to see how good a search engine deals with spam, one idea would be to search for the most spammed keywords. I selected three pharmaceutical keywords.
hydrocodone [orally active narcotic analgesic and antitussive]
Google: results #7, #8, #10 are spam pages
Yahoo: results #1, #7, #8, #10 use sneaky redirects; result #3, #4, #5, #6 are duplicate and uninformative
valium [a tranquilizer used to relieve anxiety and relax muscles]
Google: results #4, #8, #9, #10 use sneaky redirects and keyword stuffing
Yahoo: results #1, #4, #5, #6, #7, #8, #9, #10; result #2 is a site generated automatically, that uses expressions like "Is valium is for valium i was recently announced valium"; result #3 is uninformative
One page even disguised as an online poll:
Do you want?
1. cheap valium
2. buy valium online
3. discount valium
cialis [drug used to treat male erectile dysfunction]
Google: results #5 use sneaky redirects; result #3, #7, #10 are blank or nonexistent pages
Yahoo: results #7, #9 use sneaky redirects
Google wins for hydrocodone and valium, while Yahoo wins for cialis. It's clear, though, that Google puts more effort in detecting and removing spam pages from the index.