For old school weapons, I agree the Halberd is pretty sexy.
For modern weapons, I love the look of the G3A4 but i've never actually shot one. The M249 is a beastly weapon, fun as hell to shoot, same with the M240 but I remember the 249 being easier to manage. I also used to love M4s and AR15s, and they're a blast to shoot as well but they're a little too high maintenance to be a favorite weapon for me. I'd like to try out an AK variant, i've never shot one but i've seen some seriously abused ones still fire like a charm, and i'll take reliability over precision any day.
For future weapons either the sonic shotgun from Minority Report or the Lawgiver Pistol from Dredd.