The new scattershot does indeed look pretty cool...not as useful as I thought it would be though when I used it at SDCC...it only had range because of its ability to bounce off surfaces. On more open maps, and even in larger hallways it wasnt as effective as the Human shotty which had both decent range and power, similar to the un-nerfed version in Reach.
So far my fave weapon is a tie between the Covie Carbine with a high RoF, good range and accuracy, and the Covie Beam Rifle, easier to get headshots with than the Human sniper and has a good RoF as well.
The Promethean weapons were cool, and the Light Rifle is right up there with the Carbine, but it felt nice to use good Covie weapons again. The Human weapons were all kinda lame. The Human shotty and AR were really the only good guns. The DMR was not useful in mid range due to its bloom/spread and the BR had too much spread. But that was the SDCC build so I am sure it has all changed with the new Gold Build