@ OP (Original Post).
You need ask yourself an important question: "What Core do I like to play in games"
... "Soft = I can die and keep my items and play a lot with my guild mates!" or
... "Hard = I can play with a mixed group of guilds because the player base is small, but I can also solo play and not worry if you die with your gear and can continue forward!"
In the top 10 Guilds listed, I am from Kirby, we are nowhere active, but myself, Tr3vor and Kef play, mostly Hardcore, and Tr3vor is a major influence in Group runs on Hardcore, I am pretty much solo, but I have occasionally done a few two(2) man runs, even a run with Cheeya from another guild. I am NOT implying to join Kirby ... once you answer the questions, you may find a Guild that will suit you.
I viewed over a ton of the responses and can not believe that guilds are belittling each other ... John (Labattblue) is a great person, so is ATI (Ringspoiler or something like that), there are a lot of individuals from different Guilds that can provide what you see in a guild (also up to yourself and what you want out of it.)
Sorry for the tl;dr above:
Simply put like another stated, test the waters before deciding, and also gain some recognition.
Gone are the days when Guilds used to form Alliances between each other since a lot of updates, now they seem to crap on each other in posts as you can see.