Guilds are social clubs in the game, and once you've been in one that was good for you, it's pretty hard to go back to being guildless.
So, what are some factors that make a guild "the best for you"?
Here's my general list.
1) Large and active - Since guilds are primarliy groups for talking and playing together, you'll want one with very active members, probably the more the merrier.
2) A nice guild hall - It just makes for a fun place to hang out together, and having a nice one adds prestige to your guild and aids in recruitment.
3) Well-Organized - This includes clear guidelines for promotion and the kind of behavior you want to encourage as well as prohibit. A good Spiral Knights Wiki site helps a lot here, and a website is even better.
4) Playing together - Here is where people will have strong preferences. If they like guild vs. guild Lockdown, then that might be a priority when looking for a guild. If they don't want to play with newer less capable players, then they'll want an elite-type guild. If they find over-powered parties boring and like helping newer players, then they'll want a guild that focuses on that. Or maybe they'll just want to keep it to a small amount of close friends.
5) Good Guild Master(s) - If the leadership is strong, mature, and seems to always have a good amount of in-game wealth, the guild will almost certainly prosper.
6) Personality of the guild - While they are probably some guilds you know you wouldn't enjoy being a part of, you won't really know how well a guild fits you until you've been it.
7) Where you are as a player - After you've been around for awhile and depending on your personality, you may reach a place where you just want to be the founder and guild master of your own guild. Be warned - it's a lot tougher than it looks! You'll have a better chance of succeeding if you are very active, fairly wealthy, fair in your dealings with members, friendly, and accessible. When you succeed at it, there is probably no greater reward in the game.
No doubt, I'm leaving some things out that are probably important to some players and have included some things that are not important at all to others. Also, I've avoided mentioning any guilds by name, so hopefully the thread won't be about bragging about or slamming any specific guilds.
You've pretty much said everything I would've said.
Activity is definitelly at the top of my list, you've got it in one for me though.