Well if you're going for self-defense or MMA, I think boxing, muaythai, kickboxing are the three go to striking arts.
These styles have regular full contact sparring with strikes to the face. Some styles like Kyokushin Karate have hard sparring but they don't allow punches to the face, which can make a huge difference. I seen guys who have participated in tough sports like wrestling or football get hit in the face the first time in boxing turn into little girls. I thought boxing couldn't be possibly worse than judo in terms of pain, but I became a little bitch my first sparring session simply because I wasn't used to getting punched in the face.
I'm not saying other styles like karate and TKD are bad. If someone wants to learn karate for the sake of competing in karate, cool. Karate training will probably add another dimension to your game as well. But if you're a beginner whose goal is MMA, then you need to learn a style that lets you get used to getting hit in the face.
Plus, it'll probably be easier to find serious muay thai or boxing schools. Most karate and TKD schools are mcdojos that serve as a day care center. The chances are you won't find a good san shou or kyokushin school unless you live in a big city.
Ultimately, it comes down to personal preference though. If the TS is serious about learning striking, he should do his own research, check out different schools and see what he likes.
The answer usually is Muay Thai or Boxing. I prefer Boxing and the reason for that is what I am going to say to you.
Pick whatever style has the best training in your area.
The best fighters I knew growing up were all ShotoKan Karate fighters because that is like the most popular martial art period I think.
Now I know people who train in various Striking styles, Boxing, Muay Thai and Shotokan.
So When I was deciding what I wanted to do I looked at each of them and saw the Muay Thai place looked abit McDojoish e.g. Was taught in a Dance studio by an Instructor who had done alot of McDojo type Martial arts, He had sparred with former champions apparently and learnt it from a former champ who I googled and never found anything. They also had Belts for Muay Thai!! and No ring or Heavybag so I thought do I really want to train here. Also people had commented on it calling it Fitness Kickboxing and I could see there point so I didnt go there.
I looked at the Boxing gym and saw it produced Boxers who compete at a National level for Wales and Ive seen a few of them fight and they looked awesome. So unlike the Muay Thai gym full of negatives I joined the Boxing gym with loads of positives and havent looked back since.
People say Boxing and Muay Thai are the best styles so choose out of them 2 which one has the best school in your area and you can't go wrong is my advice as they are the best two so get the best training possible.
anything that has full speed and full contact sparring will probably work out just fine for you. I am really biased towards Muay Thai, but there are a lot of great stand up arts and sometimes being different can have a lot of advantages. Just make sure you do your research before you commit to a gym and that it caters towards people that actually fight.
Pick whichever gym has the best quality of training. But if you're in a major metropolis, BOXING. In my experience, wrestling is a better base than BJJ for the beginning grappler and boxing is the better standup point to start at than MT (which is also crucial but you can work on that later after you learn your footwork, conditioning and jab in boxing).
You know it's so funny to me to hear this as someone who actually did boxing as the last part of striking. Meanwhile, in the UFC, the WW, MW, LHW and HW champs (well ex-champ for HW) all have a significant boxing base.
Boxing first, then muay thai. MT is too stiff and has a lot of holes for boxers to pick apart. Traditional Muay Thai preaches a lot of concepts that are actually wrong, but this is due to their judging.
Your kickboxing game will improve significantly and be taken to another level once you understand pivots, proper footwork and movement of boxing. One of my favorite kickboxer is Michael McDonald, and the way he utilizes boxing technique in his style is just amazing. His angles, setups, the way he darts in and out are all boxing based. He incorporates his Muay Thai into it.
Any boxer who gets some basic kickboxing training, just enough to defend against kicks will pick apart a pure Muay Thai guy.