I tested Google, Yahoo, Ask.com, MSN, Dogpile, and Altavista on the W3C's Markup Validation Service to see which had the least, (or none) errors.

Here's the results:
1st place – MSN – Passed!
2nd place – Altavista – Failed, 38 errors
3rd place – Yahoo – Failed, 41 errors
4th place – Google – Failed, 61 errors
5th place – Ask.com – Failed, 64 errors
Runner-up – Dogpile – Failed, 70 errors

I would rather have better results than "correctly" laid out web pages