Over the past 25 years I've submitted coins to both services, but prefer NGC for a number of reasons. But mostly because I prefer the NGC slab aesthetics over its PCGS counterpart. I also believe that NGC is more consistent in their grading. I purchased a 16th Century English coin in a PCGS holder, and NGC would not cross it over at the same grade... I was shocked. Then I did some research and found that coins with significant provenance (like this one) submitted to PCGS in the past as part of a large estate liquidation (or other large collection submission) have been known to be less conservatively graded than lower-profile submissions. Which certainly would explain what happened in this situation. Regardless, NGC has some issues as well, I recently submitted a 17th Century English coin to them for Express Cross-Over, and it took 10 business days, not counting the 3+ business days it sat at their Receiving office because their system showed my NGC membership had expired (which should never happen to an ANA Life Member). Then when they finally logged it in at receiving, they backdated the receipt by one day. Good service? Not so much.

Both are highly credible services, but I would go with PCGS for my most valuable  coins. PCGS coins seem to be easier to sell and can bring more money when sold to dealers. When sold at auction I don't think it matters as much. I use both services and use NGC without hesitation. As for ANACS I feel that they are credible, but unfortunately their past mistakes have cast a long shadow on their reputation. I use them also, but only for my least valuable coins.

As someone who is just starting to build a coin collection, it is a bit surprising to me that most of the comments reflect a preference to the slab aesthetics as being at the top, or near the top, of the reasons.   I read a few comments noting accuracy, but it's pretty clear that particular slabs have preferences.   Seems really silly to me though.  All things being equal why would I pay a premium for something that has absolutely nothing to do with the coin?  Thanks for starting this thread because I have zero hesitation buying ANACS graded coins, that from what I have seen do auction for less than NGC coins.  Haha... just silly that a piece of plastic would significantly change the value more than reputation of the grader, and that is clearly what I am reading here.   

I think the NGC is the best, supported by the ANA, and they really seem to have their slabs up to date.

When I was collecting classic US commems, NGC holders seemed a hold a lot of very eye appealing coins... not monster toned, usually, but coins with some light color and often beautiful original skin. They seemed to have a real eye for commems, but usually didn’t over-grade. PCGS, on the other hand, graded a lot of heavily, attractively toned coins... and they carried a premium (grade and price) believe me. Not sure how it is today.

ive only ever owned PCGS slabs, but NGC slabs have some nice eye appeal.

Ok I will sum it up. If you call NGC and listen to the message. You will hear something like this. The Smithsonian Institution has chosen NGC to slab three hundred of our rarest and most valuable coins. They tested the others and they did not stand up to NGC slabs at all. I don't think I have to say anything else.

I mostly collect NGC slabs, they just seem more substantial.  I do like ANACS slabs as well, they are aesthetically pleasing and the newer ones have the type of coin on a curved top that make them easy to identify when they are in a slab box.

The main two as you probably know are PCGS and NGC.I personally like NGC the most. I think that their slabs look really cool. I also think that their grading is accurate. Another reason I like NGC is because the ANA is "partners" I guess with them and we get to submit directly to them.