Hi.
I might just be in a kinda sodium pentathol mood, but there's a few things here to look at.
Cool. Irrelevant.
But I'm not fat, my body is rather bulky (i'm not doing gym by the way).
Irrelevant.
I'm very used to cardio exercises as I'm a pro in badminton.
Cool again. Still irrelevant.
But I know that I can't be very flexible and swift, my agility is average and i'm pretty powerful.
Er… right. None of that means anything of course, being highly subjective and vague (I have no idea what "pretty powerful" might mean to you), and the definitive language of limiting beliefs doesn't bode well… but, again… irrelevant.
No idea. Nothing said so far has any relevance.
I have these options around my home: taekwondo, karate, aikido, judo, muaythai, vovinam, kickboxing, aikido.
Better, but still… not really anything to go on.
I want to train and stick seriously but I don't want to change my body (ex: become thinner).
Yeah… here's your biggest issue. You want to do something (martial arts), but don't want it to change you? Then don't do it. That's the only way it won't have some affect.
I imagine myself to be strong and tough, not really needed to be fast or flexible

that is just my imagination
Yeah, your imagination doesn't hold too much weight…
I really need advice. Thank you very much!
Yeah, you don't need advice… you have given a post littered with pre-emptive excuses. That's fine… it's normal, really. But it does indicate that you're not overly serious. I mean… you have, what, 8 listed systems/schools around you, but instead of visiting them, you ask a bunch of strangers who don't have the first clue who you are, what you like, what the schools are, who the teachers are, what the quality is like, and more, where you should go? We don't know. We never know in these situations. That's partially because we don't know which particular schools you'd have available (a simple system listing isn't often anything to really go on), but mainly because we don't know your personality, preferences, which school and instructor you'd gel with (which is of far more importance than what style it is), and so on.
You have to remember, the membership here are not all going to be familiar with all the systems you listed… and, even if they are, you aren't. You have nothing to base our recommendations or advice on, and we have nothing from you to make any meaningful offerings.
That said, if you're wanting advice, here it is.
Start. Start somewhere. Visit schools. Ask questions. Watch, or better, try the classes out. Don't concern yourself with excuses (it'll change my body… I cant' be flexible or fast…), all they'll do is get in the way. Go out and actually do it. Hell, you might find that the changes to your body are ones you like… and you might find yourself more flexible and faster than you think.
If someone tries to make you pay for your first class...just walk out. That's bad form. You try and then you can talk money.
One thing I will disagree with stonewall on though, is this. There are a number of schools that will charge for a trial lesson… mine included. I don't often discuss the meanings or reasons for the way we engage our business publicly, however I will say that it is not "bad form"… it has a number of reasons. And, honestly, they're not financial. For some, maybe… but in our case, absolutely not. Frankly, there aren't enough examples to make it a financially positive action.