Current summary about multisig.

1) BTC max 15-of-15.
Pros:
- BTC is the world's number one.
Cons:
- Group of 3-5 smart cheaters can play 15 honest people.
- In order to participate in multisig you need to publish your public key. Due to BTC design, public key becomes really public only after first sending transaction. Or you can explicitly publish it in plain text.
Right now it is safe, but in the next 10 years it will be unsafe because quantum computers will be able to restore private key from public key. Untouched BTC addresses will remain safe, but public keys - no.
(however, developers have enough time to invent protection)

2) NEM max 64-of-64.
Pros:
- It is practically impossible to write so many fake lists, someone will certainly identify cheating.
- Built-in messaging support, and public and private.
Cons:
- NEM is little-known and is slowly getting down - https://coinmarketcap.com/ - number 72

3) ETH potentially unlimited M-of-N wallet https://gnosis-safe.io/
Pros:
- ETH is the world's number two.
Cons:
- There is no 100% trust to them. ETH addresses can be blocked. And by ETH developers as they already did with ETC, and by Gnosis developers as they explicitly declare in their terms - https://gnosis-safe.io/terms

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The most promising search phrases may be:
https://duckduckgo.com/?q=simplest+multisig+smart+contract
https://duckduckgo.com/?q=multisig+dapp
(without "ETH" because there are a lot of other blockchains with smart contracts)