What is an EA? This is a piece of code that opens and close trades on your behalf.
What purpose does an EA serves? The main purpose for an EA is to save you time.
So, what is the best time range? The smallest time range available!
You see, once you start developing Expert Advisers, you'll realise that they all fail in the long run, the longer you leave it alone, the more likely your account will hit a bust. So, its safe to say "An EA is more consistant in the smallest time range", a EA operating at 1H is more likely to crash compared to a 1M range.
Why does an EA fail in the long run? All Stock or Forex Pair prices appreciate or depreciate in the long run, making it harder for your robot to adjust to those inconsistent micro price jumps. But is the short run 1M time range, everything stays or seems to be the same, regardless of the long run's direction.
What to Always remember? You will always make equal amount of returns in the short run relative to the long run. Meaning if you expect to make 20% in twenty months, you should trade with the expectation of making 1% per month. Hence when trading short, always keep you lots low and your Take profit/Stop Loss relatively low also.
CONCLUSION: I know some of you may conflict with some of my ideas, but you must understand that an EA is best relevant in the short run. Hence the shortest time range possible will give the most sustaining result for your EA in the long run (you will not have to baby sit your robot month to month).