I think the best pictures come from those topics that you know about,like, have tried them, are a part of your life...
Those pictures will probably be most realistic, as you know what details you have to be careful on. Lets say you like the army or you like trees etc. You have probably read a lot about the topic so you can imagine and manifest better pictures. If you sometimes have "photography block" as I like to call it and don't have any ideas, try to shoot topics that you prefer and you might be surprised.
In my case, I like dark,misterious and crime related photos, so I think my best pictures come from this topic. Or even more interesting if you try to combine different topics that don't fit together are not expected to be combined, you can get amazing photos that will sell.
So try photographing what you know, what you like because probably amazing pictures will come, plus you will enjoy the time you are shooting even more.
In my case:
Nothing new told probably, but if not else, it's a reminder ;) Good luck