There's quite a few subreddits that are worth looking into and it all depends on the type of content and discussions you are looking for. You can also find coin-specific communities however there's too many to list.

For Bitcoin related discussion:

r/Bitcoin
r/BitcoinBeginners
r/BitcoinMarkets
r/BitcoinDiscussion
r/BitcoinMining

For general cryptocurrency discussion and content:

r/CryptoCurrency
r/CryptoTechnology
r/CryptoMarkets

Other:

r/EthTrader
r/GPUmining

Crypto Memes:

r/CryptoCurrencyMemes
r/CryptoHumor

Here's a link to the r/CC wiki which contains a link to useful resources as well as other subs:

https://www.reddit.com/r/CryptoCurrency/wiki/directory