Is garnish meant to be eaten?
Garnishes should always be edible - there may be laws depending where you are, but in general anything on your plate should be edible or very obviously not meant to be eaten (like a skewer or a paper wrapper). Depending on the specific case, it might not necessarily be meant to be eaten, though. The defining characteristic is the appearance.

Springs of parsley or stronger herbs might not always be too pleasant to eat on their own, so there's nothing wrong with skipping them. I don't think whoever cooked your meal is going to be too upset if you don't like eating sprigs of parsley - but eat them if you want!

With lemon on a glass, you can enjoy the scent as you drink water without actually eating it, or you can squeeze it into the water if you enjoy that little hint of acidity.

So probably the best garnishes are ones that look good and are pleasant to eat. For example, a dish sprinkled with something small (finely chopped herbs or cheese or nuts or anything else that's already in the dish). Some garnishes are really all about the appearance, though. You'll have to use your judgment to figure out which is the case for whatever dish is in front of you!