This page is just my opinion, please feel free to disagree.
The two most highly repsected Third Party Grading (TPG) services are NGC and PCGS. I suggest you do not pay price guide prices for coins in other third party holders as you may not recoup that money. Currently when submitting coins to TPG's I use NGC. I don't consider either better than the other, but PCGS turn-around times are getting amazingly long (1st Qtr. 2008). I see many cases on the PCGS forum where world coins have been in for 30-60 working days before getting graded.
While you should be buying the coin, not the holder, in todays market place coins graded by other companies (or raw) often sell for substantial discounts due to their percieved looser grading standards. Personally I will look closely at coins in ICG and older ANACS holders, as there is often a decent probability that these coins could cross over to either NGC or PCGS. I don't even bother looking at the dozens of other services as, in my experience, they are so grossly overgraded as to be a waste of time.
ANACS has recently gone through another ownership change, December 2007, time will tell how this new company is percieved in the market. If you find coins in the older small ANACS holders they are often a good buy. Up to the mid- 90's or so ANACS was very conservative and used pure technical grading. I have had several coins graded AU55 and AU58 that get upgraded to MS62 or 63 when resubmitted now. The reason ... even the slightest bit of cabinet friction, or a slightly weak strike, would cause the old ANACS to give the coin an AU grade.
One new service to look for is David Lawrence Rare Coins (DLRC) Dominion Grading Service (DGS). They have recently (1st quarter 2008) purchased PCI and plan to turn it around. I wish them the best of luck as I have purchased quite a few coins from them and always found them to be very fair. We will see how they are accepted by the market place over time.
I rarely buy raw coins any more, the abundance of very high quality fakes is just amazing. The ones I do buy raw usually come through trusted sources that are considered experts in their field, even then I worry until I get them back from the TPG. My suggestion is to only buy from known sources, that slabbed MS63 Trade Dollar you got on Ebay for a bargain price, from that seller in China, is not a bargain. Not only is the coin often counterfeit, they do an excellent job on fake slabs now.