Regarding parking there's a couple of approaches.
First, you need to confirm that your domain portfolio meets the requirements of "Parking Company X". Some require a bit of traffic. Some will apparently take all comers.
After you confirm where you qualify you can either go looking for the best company OR you can sample any number by moving your domains around.
If you check the message boards you will see lots of people saying "Company Y" or "Company A" is the best. They can't all be the best, but if you check surveys, blogs, online zines, etc. you will see lots of claims that one or the other is the best. So they're all the best, right? Wrong.
Some use a Google feed. Some use a Yahoo feed. Some add other feeds into the mix. Some domains do better with Yahoo. Some do better with Google. That's why testing may matter. However, as a rule, I see fairly consistent favorable reports of Google>Yahoo from domainers that I know and respect. OTOH, IF you have a big enough portfolio you can negotiate special deals (percentage share, etc.) that may result in Yahoo>Google. It's all in the details.
IF you have more or better converting traffic you may be able to negotiate a better deal with a parking company. So, person 1 may have a 50% share and person 2 may have a 70% share of the parking company's income stream.
Here's a neet test: Look for signals of quality. In other words, take a look at the quality of the portfolios parked at the different firms. (You typically can identify parking companies by their landers OR by the DNS pointing of a domain.) Some companies have members that love to post up about their great earnings with ParkingCompanyX . . but when I dig into what the domains are that are supposedly making the money . . well, sometimes I'm just not all that impressed and I'm left with the feeling that the reports of riches are a bit inflated.
Do you homework. Shop around. Move them around. That's the best way.