In my opinion, it depends on what you have for grading. For world coins I only use NGC. I like their holder better and I think they do a better job on world coins. PCGS and CACG may be a better option for US coins, especially if you have valuable items. ANACS and ICG both have better turn a round times but are not held in as high a regard as the other 3 companies.
Micah, First you need to determine if your coin meets the criteria to become a candidate for slabbing, which includes authentication as well as grade but also special actions for label content for attribution. Take one coin: Is it worth at least $150-200? Do you fear it is a fake? Have you determined if it has doubling or RPM or any other attribute? Are you aware of the costs for having extra verbiage included on the slab label? Is there a generational/emotional aspect to be considered? Are you planning on selling this coin after you slab it? All of these questions mesh intricately with why you are slabbing a coin, and it takes research to choose wisely. @Micah 707