You didn't tell what "best" means for you. So let me only sketch a procedure:
1. Find ALL pay-as-you-go-phones offered under the conditions you prefer (unlocked, locked to Orange for a year, whatever)
2. Find a review site. Some are independent (like CNET), some shops however, also have a review option for their customers.
3. Find the reviews for all phones you found. Most result in one figure, say 8.6 on a 1 to 10 scale.
4. Pick the one with the highest score. That's the best.
5. Check the results with another review site. It's not unlikely that will find another phone best.
6. Now of those 2 pick the one you like most.