Any antique grandfather clock that is in an overall good condition and which suits your personal taste, your interior space and your budget is generally the best clock for you. You need to ask yourself if the grandfather clock you're buying is something you can really live with. The potential resale or investment value of the antique grandfather clock should be a secondary consideration.
That said, the first step in buying an antique grandfather clock is to become well-informed on the different types of grandfather clocks and antique longcase clocks, and their particular characteristics. Being knowledgeable will save you from making buying mistakes or ending up with a fake. For instance, if you are buying an antique longcase clock of Britain-make from the 17th century, it is important to check that the clock body is made of woods that were commonly used in that period; oak, to give one example. It is equally necessary to check that the clock exterior has not been tampered with or been altered in any way. Such alterations detract from the antique value of the clock.
You may also want to make sure that the antique grandfather clock you are interested in works properly. It is a good sign if the clock ticks, even if for a short duration, then with a little repair and servicing, the clock will be ready to operate. A clock that does not tick may have a damaged movement, and it may cost you a significant amount of money to get it repaired or replaced. You will need to factor this in the overall cost and consider if buying such a clock will be worthwhile.
It is a good idea to always buy an antique clock from a reputable and knowledgeable dealer who can offer you a written guarantee on your purchase. In some clocks, it may be possible to get an accurate provenance record, and this is helpful in establishing both the age and value of the clock. You can consider joining the National Association of Watch and Clock Collectors (NAWCC). This is a worldwide organization, and it is a good place to meet and confer with antique dealers and other clock collectors; a newcomer to clock collecting can learn a lot by reading the information available on this site and by participating in the forum. It will also help to make use of other online resources and visit antique and auction websites to keep tabs on the variety of antique grandfather clocks that are available for sale.