Decred is technically one of the "biggest" bitcoin dev "entity" .
I'm not saying that Bitcoin doesn't have dev support( in fact 2019-2020) bitcoin has much much more dev support than before.
Pre-2016 was pretty much just Blockstream...
Now we get Chaincode,Brink, Square, and exchanges outright granting.
It pretty crazy IMO.
But if you count Decred, they are technically also a big one. They work on BTCSuite software i.e Golang which is also used in the LND software code. It also the main "alt" node software: btcd .
Pretty funny IMO.
No other altcoin comes close to what Decred does for Bitcoin(granted Bitcoin dev do way more for the eco).