The short answer:
This is a very easy question to answer actually. I good bokuto is the same bokuto that everyone else is using in your club.
The long answer:
It depends on your practice. Some people/clubs use bokuto only for suburi (swinging exercise) practice. In this case what matters is the size and weight of the sword, if that even matters. Most proper schools of kenjutsu, however, involve light to hard sword on sword impact. This kind of practice involves a strong an durable wood.
Perhaps the most commonly used and "best" woods for bokuto is shirokashi which is a Japanese white oak. It is a dense, hard and strong wood.
A less expensive, but suitable wood is akakashi which is Japanese red oak.
Another excellent wood is hickory. This is more common in North America. It's a dense and very strong would which makes a durable bokuto.
I have to mention American White oak. It is not at all like Japanese white oak and a completely different species. I mention it because it is intentionally confused with Japanese white oak. It's ok, but I would not recommend it.
There are other woods out there that may make nice bokuto, but are largely impractical due to cost and availability. There are some beautiful woods out there that make pretty bokuto. These can be totally fine for suburi and light contact, but probably not proper kenjutsu.
As stated above in the short answer, you should buy what everyone else has. The reason for this is that disimilar would wil wear each out out much faster than similar woods. If your club uses shirokashi, get one of those. If your club uses hickory, do that. Shirokashi is harder than hickory, but hickory has more elastic strength than shirakashi. The shirokashi will cause the hickory to chip and cause small splinters while the hickory will eventually break the shirakashi.
There are also bokuto designs specific to some koryu. Jikishinkage Ryu uses a very large, heavy bokuto for their practice. Katori Shinto Ryu uses a much smaller bokuto. These koryu specific bokuto vary in length , thickness, weight and design for reasons specific to that koryu. If you practice a particular koryu, your club most likely uses that type of bokuto.
At Calgary Rakushinkan we use shirokashi bokuto and since we practice Itto Ryu kenjutsu, we use Itto Ryu bokuto.
Where to get a good bokuto?
The best answer is to ask your sense and ask where he/she gets his/hers. That way you can be sure to get something compatible with your club.
Typically I source mine from Japan from Tozando and Yamato Budogu. However, the price has gone up rather drastically so I am looking for other sources.
SDK Supplies sells excellent hickory bokuto and other weapons made in Canada. HIGHLY RECOMMEND!
I'll be adding more here hopefully...