List of generally good items you want to pick up unless you have something better
That one bird barrel - It's basically double barrel but without the double ammo cost and the bullets aim at your cursor instead of flying off somewhere else
Triple barrel or Quad barrel - Really good with certain bullets, and just generally a good barrel
Manip bullets - Really good with triple barrel and quad barrel because of the bullet's behavior, shreds bosses like there's no tomorrow.
Any bee attachment - Bees go through walls which lets you cheese normal enemies. Also extra damage against bosses.
Shotgun bullets - Lower damage, but when paired with triple barrel or quad barrel, and a high fire rate, you get an AA12