First, the main difference between the best man and a groomsman is that the best man is considered the best buddy of the groom. So even though all the groomsmen in the wedding party are technically friends of the groom,
It is the best man who has the dirt on the groom may be because they have been friends since kindergarten, or maybe because they have lived in the same neighborhood longer than their other friends. So it is appropriate for the groom (and maybe the bride as well if she is inclined) to provide separate best man gifts that are distinct from those given to the groomsmen.
Fine wine is always a nice gift on any occasion, and weddings are no exception to this rule. So find a nice bottle of wine that you think qualifies as your version of best man gifts, tie a red bow around it, maybe attach a card, and present it to your best man as a token of your affection and lasting friendship.
Still on the topic of liquor, why not buy your best man membership to a liquor-of-the-month club? Yes, there is such a creature (or service to be more precise.) Thus your best man gift turns into a series of best man gifts or groomsmen gifts because as a member of the liquor-of-the-month club your best man gets to receive monthly samples of micro-brewed liquor from American microbreweries (several bottles per month for some clubs.)
You can choose between a one-month subscription, six-month membership, and even a year-long subscription as your budget permits. If your best man is a liquor connoisseur, such best man gifts will come as a pleasant surprise. Best of all, he can always keep the liquor for the long term for special occasions, or join the club as a permanent member using his funds later on.
If your best man is a Jap food addict, then he might appreciate being presented with a collection of fine Japanese chopsticks made from different materials. The Japanese chopsticks most people are familiar with are made with wood and thrown away after one use, but real Japanese know that Japanese chopsticks can be made from other materials and come in varied designs.
You might even learn various bits of trivia about Japanese chopsticks, like that they are called hash in the Japanese language and that they are made from bamboo and metal as well as regular fine wood. Coupled with Japanese serving trays and bowls, your personalized groomsmen gifts will be top of the class and something that your best man will be able to show off to his guests in future meals.
The key to getting excellent best man gifts is buying something that the best man probably will like and may be able to use or collect. This will show your best man that you had his best interests in mind when shopping for and buying all these best man gifts to present at the wedding.