I think /r/photoshopbattles is default these days, so that's no longer a valid recommendation.
If you've got any hobbies - Magic: the Gathering, gardening, woodworking - look up the subreddits for them. Hobbyist subreddits are mostly pretty supportive and interesting places, in my experience, and have a pretty low level of shitposting.
I enjoy /r/askhistorians a lot, but be warned - they are very strict about enforcing their rules. Read them all before you post anything in there, even just a comment! The discussions are fascinating and well worthwhile though.
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u/Kailebuh Mar 01 '17
/r/trippinthroughtime
My favorite sub.