There are many variables that can come into play. First, is this a walkup with two or three or maybe four floors? Is it a small building with only a few units? Is it a secured building?
Top Floors offer the advantage of no one living above you, so you do not hear people walking above you. It also tends to be more secure, but don't fool yourself, they might get broken into as well. You will generally have a better view and maybe have a rooftop deck, though sometimes that is common space and all units can use it but you would be closer. The negative is carrying groceries up and climbing all those stairs (but you might save on your health club costs!). If you have an elevator then it is less of an issue.
First floor has the benefit of easy access for you. you may also get some patio or yard space. You may not have anyone living below you (though sometimes there is a garden unit) so you might not have to worry about someone complaining about you walking across your floor. Depending on the type of building and the access points to your unit the first floor may be less secure.
So, the big issue is what will fit your lifestyle better?