For a normal UV game (no challenges or weird medal attempts) I've found that shotgun based games to be the easiest whilst learning. With a couple of levels of reloader, even the basic shotgun starts to shine. I also recommend always doing Hell's Arena, the place is essentially made for shotguns and corner shooting, and once you have a RL your Hell Knight problems will dissappear.
A few builds that I found easy are:
Marine: Rel->Rel->BA (for the arena supercharge, MAD build later)
Technician: Rel->Rel->SM (slap your tech mod on your first shotgun, WK for modding later)
Scout: Int->Int->Rel (perfect corner shooting, maybe a juggler build for later)
Any: Fin->Jug->Rel (carry a shotgun in your current and prepared slot, switch as needed, juggle to RL if needed later).
These aren't necessarily the strongest builds, but they'll usually last throughout the game without a hitch. They also don't really screw up your build down the track. The lowering of reload times of the shotgun has made it a very nice weapon indeed.