which atv?
 

and has to sound good too

I'd personally suggest the 400x. Honda's are really reliable and sounds like you've had a pretty good experience with the 250ex aside from the fact that it lacked some power. The 400x would definitely have some get up and go. I'm sure it's fairly similar to the z400, I've just never had any experience with the z400 and I don't think you can ever really go wrong with a Honda.

The 450 would surely be the quickest of them all, but I think they're best suited for the track. I do believe however that the kfx 450 is one of the only 450's with a reverse, so that is a bonus. Maintenance on the 450's is going to be higher than either 400cc bike also.

My personal suggestion, go with the Honda 400x.

The 450 class are the same hard core track racing motors that they use in there 450 Motocross,so rebuilds will be more frequent.

The 400 class like the 400Z and 400ex are more mellow general use motors and last much longer before needing a rebuild.

Just like a 35hp Suzuki DZ400 makes a more reliable trail/road bike then a stressed out Suzuki 45hp RM450 Motocross would.

You do not get extra 10hp out of only 50cc without a consequence.

What about the Yamaha YFZ 450?
YFZ450 vs. Honda 400 EX - YouTube

Honda has a CRF 450R motocross motor in there TRX450R Race ATV also (They all have a motocross motored ATV)) what does that video prove?

The Honda 400ex is the competition of the Yamaha Raptor 350 or Suzuki 400z not the Yamaha YFZ450 or Suzuki QuadRacer R450 . Thats the Honda TRX 450's job.

Here is a shootout between the 450s.

Compare ATVs - Best 450 class Sport ATVs.

What do you mean by maintenance? I would like to work a bit on the quad I'm going to have, but not too much, just so I could learn something. Don't want to work on it before everytime I'm going to ride. Is a 450 like that?

Btw, I'm wondering between TRX400, Z400 and YFZ 450X now. Reason why I added YFZ is because the motor sounds good, looks extremely well build, lots of accessories parts, looks the best and has better stats than both in everything, except reverse gear.

I know that Honda and Yamaha are great bikes because I own a Honda and a Yamaha, but is Suzuki the same good?

well- since the only prereq's are:
"something that has good durability (that can go through slow rocky trails), looks cool and comfortable, is not slow, is good in corners, sounds cool and something you'll love..."

then I would buy a can-am renegade 1000.

1000cc is too big for me.

I,m a motocross fan, since the sport 450 ATVs all have the same motors as the 450 Motocross motorcycles use then it only makes sense the will need the same 150hr to 200 hr rebuild as motocross bikes need as well.

Remember the 450 motors were made for the Motorcross bikes first,then they just added them to race ATVs later.