I guess it depends on where you’re from. As a German I only use the 3 biggest German NZB boards which are also free to use and they all redirect us to the downloads of the indexers mentioned. That's why I assumed they were the biggest.
I just found out they're not and that there's way more popular indexers which you have to pay for which is hilarious to me.
I use Premiumize, multihoster for most popular Filehoster with access to Usenet, Torrent and VPN. Usenet retention is about 1200 days. It has 1TB of Cloud Storage and you can use their inbuilt Downloader on their website to download stuff straight to their cloud (or your own Mega or Google Drive cloud if you prefer that), so you don’t have to install any Torrent or Usenet clients. (This is not an ad, I just really like their service and been using it for 3 years)
Sabnzb or NZBGet are good choices if you wanna use some client on your own. Just don’t use Momentum, they will steal your credentials.
Most popular indexers are NZBIndex, Binsearch and NZBKing but I prefer Boards like nzbgrabit, nzbforyou or The Hive.
Tiny face goes :3