/mlp/ is 4chan, so it's great for learning how to be an asshole for no reason and get random bans also for no apparent reason. Stay there only long enough to thicken your skin. It can be addictive since the boards move fast enough that there always seems to be something new going on, until you realize that "something new" is more often crap and hopeful expectations for something worthwhile.
ponychan is like /mlp/ split into several subboards, and fewer users staying anonymous. It had potential for a while, but like 4chan it hasn't aged gracefully.
Round Stable/ponygoons is an odd mix of some who are as juvenile as your average 4channer and some who are more level-headed. They're militantly anti-clop, you can't even joke about anything adultish related to pone around them. Other than that they're not too bad from what I could tell.
I hear Derpibooru has a community but I know nothing about it. I just use that as one place for finding images.
EqD is like the previous Anon said, good for news. The comments section is about as bad as Youtube's.
DerpyHoovesNews is EqD's angry retarded kid brother, unfortunately.
MLP Trading Post and MLP Arena, though open to all pony generations, are mostly friendlier to pre-FiM fans. They're both more traditional BBS type boards, not like the chans. Some nice users, some assholes, and both suffer from staff in the latter category.
And there's here. /oat/ is a bit like ponychan with its cliques and circlejerks, but the mods don't instaban you if you don't assimilate, so the inevitable drama that flares up is often left to run its course. /pony/ is glacial with most of the traffic being in PAD threads, which are sort of like a mix between /oat/ and the old /b/reads. /anon/ is sort of post-4chan, with some hold-outs still so brainwashed by /mlp/'s rules that they flip their shit if someone doesn't adhere to their utopian view of "board culture," and others who've grown up. /anon/ also has clop if that's your thing. Haven't spent time in other boards here to say anything.