The best response time, theoretically, is 0. The lower the better. However, the lowest you are going to find on consumer monitors is 1.
Before I go any further, there is a LOT of misinformation about response time and what is actually is...
It is NOT the time it takes for the actions you do with your mouse/keyboard to be represented in game. That is input lag.
What it is, is the time it takes for the pixels on the screen to turn from one colour to another.... and this is where it gets tricky.
There is no standard for response time measuring. Some companies measure how long it takes for a pixel to turn from Black to White, some from Black to White and back to Black again, and some go from Grey to another shade of Grey instead. The...