It has been my experience that parking hasn't "ruined" my domains as far as search engines go.
There is no generally accepted answer to the "best parking site/company' question. There is little transparency when it comes to domain parking PPC revenue share (from the upstream) or revenue sharing (with domainers, who have varying quality portfolios).
The 2 main feeds, Google and Yahoo, reportedly offer advantages to domain subsets. I don't know. I've stayed with a Google feed.
Another important issue is whether and how you can optimize your own domains landers, and the attractiveness and flexibility of the landers themselves.
Like SEO, conversion science, PPC, affiliate programs and pretty much everything else the answer, invariably, is something like "test, test, test".
Tip: Once you collect the names of the various "domain parking" services you can get an idea of the type of quality - or crapola - domains the service attracts/allows by looking at the DNS records reported by DailyChanges.com. For example, I've noticed that one supposedly "big" parking company has, for ~year, kept its total domains number inflated by adding a high percentage of .info domains. Another parking company "with big numbers" continues to add domains that . . just feel icky. Looking at DC gives you an idea of the company you will be keeping.