Credibility problems is a great guild, as is In The Jelly and Is A Vampire is ok when Cocobe and Patches are active. Those are all more "community guilds" which aim to build a community of people. Then there is Shard squad, which is the self proclaimed "hipster guild" that my friend Zeddy runs. Its a ton of fun if you like using the more unconventional weapons like shard bombs and winmillion. I don't know much about lockdown guilds, so disregard this if you want to go do that or whatnot.

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Credibility problems, few rules+anarchy(contains many newer players, some older, some beggeary people): Meeji
In The Jelly, community(contains many older players): Dancinjin, Milkman
Elegance, community(good mix of newer/older players): MysteryZX, Hollows, Soral
Is A Vampire, community(contains newer players, some older): Cocobe, Lord-patches
Shard Squad, hipstery weapons(a good mix of new and old players): Zeddy

Those are a few guilds I'd consider worth checking out... you can try some of the other "big name guilds" but consider that for Jempire (which is huge) you'll need a mic+ventrillo to get in, whereas Echo of Silence is very exclusive and doesn't accept a great number of players.

Most other guilds are either dead/inactive+trying to fix that and you could help if you wanted (is A vampire is like that, but they're doing a good job), or are really new and trying to get a hold on things, so be sure to do some research/talk to them.