The safest to use, the best bang for the buck, and just as effective as anything else, that's what you get with Southern Ag Natural Pyrethrin Concentrate. First of all, it's a concentrate. You mix a cap-full or two into a quart spray bottle and then add water. 8 ounces of concentrate is going to equate to about 20 quarts of mix. That is an incredible value. It's made with natural pyrethrins, which is an insecticide made from the chrysanthemum plant. It's water soluble, biodegradable, and about as safe as a spider killing insecticide can be. Pyrethrin works to disrupt the neurological function in a spider, so death is relatively quick and painless. This is the best insecticidal spider killer there is, and it's so versatile that it can be used as a generic insect control substance. Great stuff.
From a name you know and trust, TERRO Spider Killer Spray will get the job done. It is made with two synthetic pyrethrin ingredients, which act as neurotoxins, so although not natural, it still breaks down and is relatively environmentally friendly. The carrier is a mineral oil based substance, so once sprayed, it offers a residual effect that keeps killing spiders for up to 12 weeks. It comes complete and ready to use in a can, spray it on webs or where spiders are lurking, and let the active ingredients do the rest.
For the longest lasting residual effect in a spray, choose Hot Shot Spider and Scorpion Killer. It is made with a synthetic pyrethrin derivative, that is mixed with two types of carrier oil substances that keep the active ingredient working for up to 16 weeks. Although this is one of the more expensive spray type of spider killers, if you have a severe spider problem and want to keep them from coming back, this may be the best alternative for that. Spray it on webs, on known spider paths, in cracks and crevices for unrivaled residual protection that can't be beat.
This may be the best spider killing spray that you've never heard of. Spider Not Aerosol Kills Spiders, true, but it is also packaged in a highly pressurized spray can that will easily reach high up on ceiling or corners where most sprays won't go. It will actually kill any spiders as the blast from the can will knock down any webs if they are present. The active ingredient is a synthetic pyrethrin named D-Trans Allethrin. Although like most synthetic pyrethrins, it is relatively safe and not very toxic to humans, this particular formula is very toxic to cats. The petroleum distillate oils it is mixed with give it a very high residual effect, and although safe and effective in most situations, if you have cats as pets, use something else.
For the best deal in a spider killing spray, get Black Flag Spider and Scorpion Killer Aerosol Spray. Made of a synthetic pyrethrin in a mineral oil mix, this spray kills as well as anything else and lasts up to 16 weeks of residual effectiveness. If we were giving a bang for the buck award for a spray can, this one takes the prize. Realistically, every other spider killing spray kills spiders just as effectively, but Black Flag costs less then any of the other spray types on this list. It is cost effective to use, lasts long and you won't go broke if you need a couple of cans for a major spider infestation.