best URL format

What is the best URL to be used?

The shortest sounds the best.

is example.com/webmeet2013 better than example.com/web_meet_2013.htm
is example.com/architecture/web-meet-2013 better than
example.com/web_meet_2013.htm
is example.com/arch/web-meet-2013 better than
example.com/architecture/web-meet-2013
is example.com/web-arch/web-meet-2013 better than
example.com/building-arch/web-meet-2013

all such examples
-- 
Sincerely,
Ganesh J. Acharya