As others have noted, there can be significant differences between schools withing the same system. I'll offer some general info, along with my qualifications to comment on each art.
BJJ (I am a long-time practitioner and instructor):
At any legitimate academy, you will learn highly sophisticated and highly effective methods for ground grappling. You'll get plenty of live sparring/grappling. You'll get a pretty good whole body workout from all the live grappling.
Depending on the school and the instructor, you may or may not get a decent amount of time devoted to supplemental conditioning, takedowns, takedown defense, stand-up clinch work, defending against strikes, using strikes to set up your grappling, street self-defense applications, MMA applications, sport grappling applications, or guidance in distinguishing between applications in those different contexts.
Aikido (I have not trained Aikido, although I've watched a lot of it, trained with some Aikido practitioners in other contexts, and practiced a number of the techniques from Aikido which exist in other arts)
Hapkido is derived from Aikido, so much of what you'll encounter should be familiar to you. I don't think the vast majority of Aikido schools work to develop effective fighting ability in their students. However if you already know how to fight, I think that the art contains a number of useful principles.
Arnis (I haven't trained Modern Arnis specifically, but I have trained various other FMAs off and on over the years and from what I've seen of Modern Arnis it doesn't appear too different)
You will learn plenty of good techniques and principles for using weapons (probably sticks to being with, but possibly knives, swords, and empty hands using the same concepts).
Depending on the school, you may or may not learn the difference between high-percentage applications, low-percentage applications, and developmental drills. You may or may not get in a significant amount of sparring. You may or may not get a significant amount of physical conditioning.
Wing Chun (I have a bit of WC training, but I am not a high level practitioner)
You can learn some cool concepts regarding structure, sensitivity, and control of the centerline. You'll likely spend time practicing a very small number of forms and in doing partner drills.
Depending on the school, you may or may not get any significant amount of sparring or the understanding of how the concepts of the art play out in actual fighting.
Ninjutsu (assuming you mean one of the Takamatsuden arts, I have about a decade of experience in the Bujinkan)
Lots of very cool and legitimate concepts and principles. Unarmed striking, grappling, weapons, and tactical thinking.
Unfortunately, this also comes with a training methodology which doesn't do much for developing actual fighting skills. You'll find a lot of instructors teaching based on theory without an understanding of the difference between high-percentage and low-percentage techniques or of how the concepts of the art will play out in real-world application.
Karate (I'm not a karate practitioner, although I've trained with some karateka over the years)
Kata, Kihon, Sparring, One-steps, Bunkai, Conditioning. My impression is that the ratio of those different elements can vary widely between schools.
TKD (I did a little bit of TKD a long, long time ago)
You'll get really good at kicking and probably get a great workout in the process. If it's a WTF school, it may be more focused on Olympic-style tournament sparring rather than martial application. Otherwise, my comments above regarding Karate apply.
Bagua
I have zero experience with the art and have no advice to offer.
Muay Thai (I've been training off-and-on for a couple of decades, have an instructor's license and a couple of amateur fights under my belt)
Solid striking and clinch work. You'll probably spend a lot of time doing pad work and bag work.
Depending on whether the gym caters to casual hobbyists or to aspiring fighters, there can be a significant difference in the amount of conditioning and sparring and in the quality of instruction.