you cant really have a best martial art, nor can you make comparisons between the two because it depends on the person. for example, if the bjj guy is way better at his game than the tkd guy is at his, chances are pretty good the tkd guy will get taken down and submitted. but it also goes the other way.
if the tkd guy is better at his game than the bjj guy is at his, the tkd guy will have the bjj guy knocked out before he can touch him.
but problem with TKD nowdays is that there have been alot of these things called "mcdojangs" put out there. specificly the ATA taekwondo organization. it is litterally a chain of taekwondo schools, the equivalent to a mcdonalds in the resturant industry.
no quality and just in it for the money.
if you are looking for practical tkd, it is 100% up to the instructer weather he wants his students to learn practical marerial or not. WTF taekwondo is the olympic style taekwondo (no face contact with punches, no throws, no takedowns, elbows, or knees) obviously not practical, right? BUT,
if your WTF taekwondo instructer doesnt give a rats *** about the olympics, then you will be learning material simmalar to that of ITF (traditonal) taekwon-do, which i will talk about in a minute. if you have a good instructer, you will be learning elbows, knees, punches (face as well), kicks, joint locks, throws, and takedowns. weapon defense (guns knives and bats) and depending on the school, the instructers will sometimes have a blackbelt in hapkido or BJJ (like my TKD master does). they will incorperate the grappling into classes or have a totally different class for bjj. I got lucky so i get to do both all the time.
i talked about WTF tkd so this is what ITF tkd is: it is the traditional kind of tkd, used for the korean military. it includes punches, kicks, elbows, knees, joint locks, takedowns, takedown defense, standing submissinos, and weapon defense. but once again it depends on the instructer. WTF and ITF tkd can be very simmilar depending on who is teaching it.
ok now bjj.
bjj is awesome for one on one encounters. not so much when you are getting mugged. when you have 3 or four guys coming after you with weapons trying to attack you, you can have one of them in an armbar while the other beats you over the head with a golf club. so this is where i suggest to you that you not rely on bjj for self defense. you cannot handle three or four guys on the ground alone unless you are superman.
as for who would win a fight between two blackbelts, like i said, it is impossible to know. they are two tottaly different people. each have different strong and weak points, and one may be more mentaly and physicly stronger than the other.
PS. im sure what you saw online was WTF tkd. go on youtube and look up ITF tkd sparring and you will see the difference. and well taught tkd is not flashy, at least it shouldnt be
6 years of taekwondo and BJJ