For sales Sedo will probably bring the most offers. For income I use to prefer yahoo feeds like Parked, TrafficZ, SmartName etc... as they allow more customization but with the yahoo/bing merger I have been dusting off my google accounts and consider 2011 a whole new ballgame. Personally I'm gonna test Sedo, Domain Sponsor and possibly NameDrive and Fabulous on the google end and compare to my yahoo feeds I have been using as I think any opinions I have from previous years of usage don't hold weight anymore so time to act like I'm brand new and get fresh results. There is no best parking company on a whole just a best company for each type of domain and traffic source.