#1 Cedrus atlantica - Dump - send to ex-wife. Cedrus do not back bud, nothing interesting in the first 20 cm, first 8 inches of trunk. Nothing will ever sprout there. Don't waste time.
#2 - 2 non-descript junipers. - okay, no loss if you dump them, BUT, these will back bud with good horticulture and full sun. Junipers are the silly putty of the bonsai world. If you prune back to almost nothing, wire and twist up what you have left, let them grow out 5 or more years, repeat, within 10 years you could have a nice pre-bonsai ready to begin styling. There is much better starting material out there, but these could be rehabbed as a long term project if you have space.
#3 - Pieris japonica - dump the one with 2 fat trunks and one slender trunk. The one with 3 trunks of different diameters, could be worked into a 3 trunk clump. Pieris do back bud on old wood, though not abundantly, they will occasionally back bud. These like many flowering trees are raised more for the in bloom display, or the autumn color display, not because they are particularly tree like. Raised like a kusamono, where the kusamono itself would be the focal point of the display. Keep the one.
#4 - Picea pungens 'Hoopsii' - this is named as if it were a grafted tree, though I don't see the graft. You have nothing going on, no surviving branches in the first 8 to 10 inches of trunk. BUT, you do have branches a little higher up that are not quite dead. You need to save these lower branches,. Once this recovers health, full sun is the only cure, sunrise to sunset full sun. Once healthy, it will back bud on the branches though probably not on the trunk. You can bend down, wire down branches to create a believable tree even though you lost the lower branches. Dump it if you want quick results, it is at best another 10 year project to turn into bonsai.
#5 - Picea pungens - Dump it, no character in the first 8 inches of trunk, no low branches, not enough age to make ''literati'' there is nothing useable there.
#6 - Thuja occidentalis 'Rheingold' - as in the Cedrus, these do not back bud reliably. You again have lost the lower branches, and the close to the trunk living foliage. Dump it. However, Thuja will propagate by cuttings. It is a nice cultivar. If you like you can use this tree as a donor for propagation by cuttings, or air layer a top piece. Otherwise, if you are not interested in propagation, dump it.
Key to selecting good material for bonsai is the first few inches of trunk need to have an interesting feature, nice bends, or twists or unusual bark or large diameter. You normally need branches low on the trunk and foliage growing on the branches close to the trunk.