I think those posts which help you solve a given issue are most worthy of a rep. Imagine I post about a small snag in the codework of my forum and someone comes up with a fix for the issue. To me, it is a rep worthy post.
As Saru said, those are posts that helped you solving a problem or fixing a issue . Or a tutorial, a good tutorial or guide is always worth some rep 😀.
I think that posts should get rep if they genuinely help people out and if the post is quite good when it comes to answering a question or a good post in a debate is worth rep for me.
I usually rep posts that offer new arguments to an idea that are well presented. If the new argument has some comedy behind them, I'll be more likely to rep them.
I do hate it when people give reputations to spammy posts or otherwise posts made by good friends of theirs. I believe that this should be reserved for primarily well-worded or otherwise highly agreeable posts.