1: Rob is telling the funniest joke, you should hear it
...is actually quite natural, but so is...
2: Rob is telling the funniest joke, you should listen [to it]
The difference between hear and listen is similar to that between see and look. In both cases, the second verb implies a deliberate act that involves paying attention - hence #2 above more strongly implies stop talking yourself and/or listening to someone else, and pay attention to the joke Rob is telling right now.
But if we shift to the past tense...
3: Rob told the funniest joke, you should hear it
is also perfectly natural (Listen to me now, while I tell you the joke I heard from Rob). But...
4: Rob told the funniest joke, you should listen [to it]
...is at least slightly unusual. Maybe the speaker means the same thing as in #3 above, maybe he means you should go and ask Rob to tell the joke again, or maybe he means just find anyone who heard the joke and get them to repeat it.
Note that the word it is syntactically required with hear above, but to it isn't required with listen in #2, where the intended sense is listen right now (to the joke that Rob is currently telling). Nor is it required in #4, if we're in the relatively unusual situation that the joke is about to be retold.