Bitalk.org -> Seems to be Russian only.
Bitcoin.com -> Not too active (not bad UI).
litecointalk.io -> Not really interested in a Litecoin specific forum + rather inactive.
cryptocurrencytalk.com -> Rather innactive on substantial threads (like the video section).
bitcoingarden.org/forum/ -> Very similar UI (SMF). Structure a bit weird, and 24K members only.
bitcoinforum.com -> Also very similar UI (SMF). Not very active, and just under 18K members only.
allcryptotalk.com -> Not many boards not activity.
cryptorum.com/forums/ -> Very weak in terms of boards and content.
talk.peercoin.net -> Specific to peercoin
mastersofcrypto.com/forum/ -> I like the guides .. but nothing else (+ barely any activity).
So, in summary, none of the above come even close to Bitcointalk neither in terms of content nor, very likely, prestige and history of its members.
I don't think any other forum is worth visiting and this summary is accurate. At least the OP didn't put that phishing .io clone either.
Every time you search things online, you inevitably find links to this forum, even altcoins need to born here is its like they never exist.
Reddit is sometimes interesting, but the only thing like this forum is of course the official #bitcoin channel at
Freenode.