Favorite gun in any cod? Scar-H from MW2. Lazerbeam with good damage (like all guns in that game) and better iron sights than a bloody red dot? Yes please.
However in my ideal world, the guns from black ops. They had more recoil - so they wouldn't be as good as say mw2 guns in comparison - but BO is by far the best game in the series because of its gunplay/maps.
Recoil and multi-ranged engagements meant you had to learn to control your gun, either by adjusting to the recoil, or by burst firing.