Another dimension you can use if the already mentioned ones are unavailable are what did the developers work on most recent. Or can you provide instant feedback to the developers so they do not have to context switch. I.e. the priority is unclear or not that critical at this stage and the developer just worked on fixing a nasty bug. Go for the bug because if you find anything there they can jump on it straight away.
James Bach introduced a concept that he called shake’n’bake (no idea why) which basically meant you as a tester go to the developers computer and test their stuff in front of them. Two things happen here. They get to see what they can do to do better testing themselves and they can fix any problems right away. In our case something that takes 2 hours now take 10 minutes which is a huge time saver.
Just to be clear, in most cases you should prioritize your work based on importance before you go for avoiding context switching.