too subjective. if you find that you're spinning out a lot, add two teeth to your chainring. experiment until you find what works for you. you'll notice that it will be harder to accelerate. if you ride in an urban area where you need to stop and start often, a bigger gear might be slower, or it could cause more fatigue. that will be slower in the long run.
do you want to feel faster, reaching higher speeds for the thrill of it, or do you want to get from one place to another faster? there are too many variables to give you an answer and anyone who can prescribe a specific number for you is full of hot air, but those are the kinds of questions you need to ask yourself to figure it out.