I just started learning about ark and it sounds really promising. I don't know much about spark other than a lot of people complaining about it since apparently whoever's coordinating it ultimately controls your Bitcoin (I think).

I think cashu and fedimints are really cool. I think Liquid's got some potential as well. I have played with Liquid in the past and I still have some LBTC on a wallet.

There will be multiple layers for a Bitcoin standard, so it's interesting to see how they all play out. The key will be balancing out the benefits (speed, cheapness) with the risks (whatever degree of custodial relationship another party has).