In practice, once you get experience with grilling you won't need a thermometer, you'll know the meat is done by feel, or by lifting it with your tongs and seeing how much it bends. (cooked means harder, less flexible)
Anyway there are two types of thermometers that are worth owning.
I) A decent Instant Read. Tons of brands, most of them are good. the more you pay, the more "instant" it is. $20 buys you something that will take 5-10 seconds. $50 is 3 seconds. $80-$100 is more or less instant. In practice the speed is cool but doesn't make much difference. No real need to spend a lot of money.
Two brands I own and like:
$20: CDN Pro-accurate instant-read.
$50: Lavatools Javelin Pro is faster, well built, longer probe. I own it but it's not really a big step up from the CDN for being twice as expensive.
2) Remote-probe thermometer. You stick the probe in your meat and you can monitor the temp real-time. These tend to wear out fast because the probe or wire is easily damaged. I have found the CDN brand to be long-lasting. But there are other brands that get good reviews so I assume they're good.
Note, if you want to use Amazon reviews, you HAVE to read them because so much is bogus.. A lof of time the stars are completely misleading or wrong -- because the reviews are for a different model or they're just plain fake. So.. use the reviews but with caution.