I haven't looked at the code. From the white paper it looks like it has what you are calling a "Zero Knowledge solution" based on the methods from cryptonote.
Monero and Darkcoin are different projects.
Monero uses cryptonote, so it has what you are calling a "Zero Knowledge solution" (and has had it since day one, in April 2014). Nobody outside of the SDC community calls it that, but you can use whatever terminology you like I suppose. Cryptonote (i.e. Monero) does use a simple zero knowledge proof in its signature scheme, the same method that SDC uses.
Darkcoin uses a variation of coinjoin where inputs and outputs from various transactions are combined by a series of semi-trusted nodes called Masternodes, essentially mixing those transactions together. There is not really a special transaction type for this, just a regular bitcoin-style transaction with many inputs and outputs.
Here is a Monero transaction:
http://chainradar.com/xmr/transaction/ab187945caff3a9ca5034338657808f242d8f2899fc11d93f2ef2d719bb16ecd
I don't know enough about the others to provide correct links to anonymous transactions.
What many people seem to be missing (and it is totally normal to do so because it's not been seen before) is they keep comparing it to existing solutions.
SDC has something completely new in that it destroys coins that are then converted into redeemable tokens that sit on top of the blockchain and these tokens can then be reconverted via minting into new SDC.
It's only the exchange / allocation of tokens that use the ring sig with basic NIZK to prove to the blockchain that the holder owns the value of the output without revealing identity, it's not the actual coins using ring sig.
It is indeed a very well thought out system that cannot be followed on an explorer because it'll just lead you to a dead end. Hence 100% anon.