Anything muay tai
Both are equally good martial arts.
Anyone who says different doesn't know anything, for the simple reason any disaplined martial artist would respect other arts.
Some taekwondo dojos are geared around tournaments like olympics labelled sport Taekwondo which will be mainly kicks which is high intensity as you cant really punch in tournament as such...a waste if you want for self defense great for fitness so check you learn full taekwondo!
Muay Thai is very much like kickboxing which is not usually as disaplined so imo not as intense training but usefull for self defense as it simulates real life more.
Choice is yours....I have done both and now do tatsu style kickboxing.
This is my usual martial arts warm up/workout before I even start the class..
These are also good workouts to do at home..
And lasty every night do this (only 4 mins)...
Every Night Of The Week Challenge
Also follow a guy called Mario P Canel on here... who creates amazing martial arts workouts.
...and also has many motivational quotes to keeps you focused.