Does anybody know what the best SEO practice and most effective format for URLs is currently?
All lowercase or mixed case? Example:
Search engines don't care about case.
Rather than navel gazing about whether you should use mixed case or all lower, work on real search engine optimization, which is creating content that give people a reason to visit your site.
Search engines do care about case.
Capitalization doesn't affect your rankings directly; however, if you have links to a page both in caps and lowercase, the search engine will view it as two distinct pages. You begin to run into duplicate content issues. Depending on your server, example.com/Page1 and example.com/page1 are two different pages, but search engines will always view them as separate URLs.
From a usability perspective, use lowercase. As a user, I don't want to pay attention to capitalization when I'm typing in a URL.
The way you show the URL you gave as an example, the "what is my site" part would be a directory, not a file name! Also, regardless of what these fools told you, you should always use lower case when specifying your URL's. Likewise, stay away from "dynamic" URL's (URL's created with scripts or programming languages such as PHP of JSP), instead use text readable URL's without all kinds of "+" or "?" symbols that no one including search engines can hardly read.