Okay I've been wondering this for ages: how the *** do you get a giant cave spider to spew webbing on specific tiles? And wouldn't the dwarf it was spraying just get caught by the trap, thereby ruining the setup?
Read the Wiki on Silk Farming.
Wiki on silk farming is out of date, I'd update it but I'm not entirely sure how, tame GCSs are totally ok with webbing invaders now as long as they're in range, they don't bother with wild animals yet though, even wild hostile sentients like cavern trolls. I didn't manage to test FBs or megabeasts when I figured this out, and also I didn't have the opportunity to see if elves are exempt from webbings because of their whole alliance with nature thing, but naked goblins behind fortifications work as bait, as long as they're unrestrained, just give them a door to run to with another cage trap by it so the spiders spray a good portion of the cage traps.
As for my opinion on the best trap, I find cage traps so effective that they feel cheaty, they seriously need rebalancing cause get a couple cage traps around your entrance and you basically don't have to deal with megabeasts unless it's an FB or a titan, or if you made them outside then a roc, and you use the same cage for a giant elephant as you do for a cavy. So best in terms of effectiveness, sure, cages, but best in terms of having fun, depends on where they are, how much fun you want, and what materials you use. I tend to go for ordinary weapon traps, silver spiked balls, make things go splat. Generally you don't have bits flying off so you don't need to do as much in the way of cleanup, and it's fun when things get propelled by the force of the blow into other weapon traps. It's not so effective against big bois but they only usually come one at a time, so they tend to get hit by all of the traps, making them more manageable for my militia, or they're of the randomly generated variety, in which case, I wish my militia good luck.