I'm not interested in any of these coins to be fully honest; I just want to see which privacy concepts exist, what are the upsides / downsides, and which are best suited for Bitcoin.
The most inline with Bitcoin design would be the Mimblewimble chain format because, unlike other designs discussed in this thread, it achieves better privacy by making the protocol simpler. It also comes with the simplest mixer which, as far as I know, is much more efficient than any mixer on Bitcoin.
I don't think Monero's ring sigs are worth considering at this point. The idea was very interesting years ago, but at least to me, it seems like a relatively bad tradeoff to make today. You're much better off adopting ZCash's newest z2z variant or something like a variant of Lelantus.
Btw, regarding drawbacks listed in Grin. Interactivity is a tradeoff rather than a drawback. Let me list some benefits of interactivity that are overlooked:
1. It allows one transaction flow to create all possible transactions (you can't build a payjoin or other multiparty tx with a noninteractive transaction)
2. Payjoins could in theory become the default behaviour (read more about payjoins here
https://en.bitcoin.it/wiki/PayJoin)
3. Any transaction party can decide to bump fees if they want to speed up transaction inclusion in a block
4. Any transaction can provably commit to a document (with a multisig)
5. No more need for a test transaction before sending the money
6. In MW, the receiver proves the ability to spend the output when they receive it. It becomes impossible to send to an address whose private key was lost
7. Parties can pay for their own onchain objects e.g. if the receiver wants to create 7 outputs, they can do so, but they pay 7*output_fee + 1/num_participants*sig_fee
Benefits of interactive-only transactions (Mimblewimble design):
1. Every transaction comes with a cross-input-output-signature-aggregation
2. You get full wallet control - unlike in Bitcoin and other cryptocurrencies where you control only what you spend, you can actually control what you receive
3. No more taint/dust/ads output injection attacks
4. Potential for the unification of onchain and lightning transaction flows (or at least making these very similar)
5. You know which outputs you own can exist since you've created them. This means there's no need to scan the blocks for your outputs if you don't reuse the seed on multiple wallets e.g. a hot wallet on the mobile phone.
6. Since you create the outputs, it becomes possible to label the outputs at creation. Whether that's some kind of graph coloring, note keeping or something else
Drawbacks:
1. Cold storage becomes tedious
2. Exchange integration becomes more painful
3. Interactive experience (not necessary to be online at the same time)
Note that if drawback 3. is so bad that it can't be widely adopted, this means that neither can the lightning network because it also requires interactivity.
Point 1. and 2. are of such nature that a solution needs to be built once so it's really a O(1) cost and after this, you're good to go.