The best pads for a Flex 3401 are the ones that are larger in diameter than the backing plate. Kind of like the best wax for a car is the kind that is derived from the leaves of a Brazilian palm tree.
My point is that you will get a ton of different answers for this question. I have a lot of different pads that will fit the Flex 3401 and it depends on which particular paint system, color of paint, size of panel, degree of correction I am shooting for etc. etc.
There are too many variables to what can be done with a polisher to say what is the best pad. Like with wax, it will be purely be a matter of opinion. I happen to reach for a Lake Country 6 inch variable contact pad most of the time, color depends on what my intentions are at the time so I'll say that whichever pad gets the job done is the best and I may go through 3 or 4 different type or brand of pad on a test panel to gain the knowledge of which is best for the job.