I definitely concur that Hackers and Founders tends to be great as a "hang out" style event – don't go with expectations or a goal in mind, it's very much about meeting interesting folks, probably more than a little tech/startup heavy for mere social gatherings.
From there, it really is going to depend a lot on personal interest – you can meet all kinds of interesting people, in a context you don't understand/appreciate, and not mesh well with the group/individuals as a result. For example, if you really hate hiking and camping, it's going to be really hard for those meetup groups to enjoy YOUR company, even if you appreciate theirs. 😉
That said, one great way to meet interesting people is to look through the available interest categories, find some interesting groups, and start reading up on their members/organizers. Ask a question or two. Then attend those events where you would have an interest in the topic/theme even if you were the only one in attendance, and then revel in all the bonus cool people who show up.
One other great place to look is in the meetup.com forums. Especially the organizer forums – you can very quickly identify like minds or just fascinating ones based on how they discuss/query/argue various topics within a given topic area, locale, etc.