I will always buy the top floor flat. My reasons are follows.

a) In most cases you will have an unobstructed views:
b) Less noisy from the street
c) security.
d) Noise from other flat owners using lift, staircase, corridors etc.
e) Will not be disturbed by the moving of furniture, stomping and high heal shoes etc.
f) Can come with large terrace etc
g) cigarette buts will not lend in your terrace.

All modern blocks will have a lift so elderly visiting is not an issue. It for this reasons that you find they top floor flats are the most expensive.