I'm not too sure what you mean by activity. You might be surprised to learn that your guild has more grey dots (active members) than a lot of the top 100.
I did laugh recently at an FS Box that recently claimed to have at least 15 members online all the time, only to check and see only two that were active in the prior 20 minutes and at least one on autoreload. The person advertising was in fact, not active.
Some people really think it's important for a good number of their Guildies to be on when they are. For me, it's mostly irrelevant, but I get the logic.
Anyway, personally I would suggest:
First, decide what game aspect(s) you most enjoy and try to weed out possible options based on that. If you are undecided, then simply look for a Guild that caters to that.
Second, decide what "activity" means to you and how it fits into what you're looking for.
Anyway, hope the suggestion helps.