Best tripod brand for a Nikon DSLR camera?

I have a Nikon D5000 and I'm now looking for a tripod for photographing portraits, waterfalls, and landscapes. I have been a photography since I was 14 and I'm now 16.

Any tripod.seriously.
That camera isn't very heavy at all (compared to most other pro Nikon DSLR's) so seriously, any tripod will do. Just go into a camera shop or electronics store and pick one you like.

Picking the "best" tripod can be tricky. There are a few brands that tend to be good, and it is hard to say for certain which is best. Bigger is better in use, but smaller and lighter are easier to transport. You want to spend enough for it to be good enough, but I would expect that your budget is not enormous. Ball head vs 3-way? Many find ball head more convenient, but for portraits I think that a 3-way gains the advantage.
This one is on the heavy side, but it should work well:
http://www.bhphotovideo.com/...ripod.html
The quick release plate for the Manfrotto RC2 mount has a plastic pin that is supposed to be helpful with video cameras. For stills you can push it out from behind or pluck it out from above. It can be reinserted if somehow you ever need it (I never do.)
This one is a little bit lighter and more compact, but should be good and steady. It has less height.
http://www.bhphotovideo.com/...ripod.html
Even lighter and less expensive, if it is not too windy this may be adequate
http://www.bhphotovideo.com/...ripod.html

Just make sure that any tripod you buy is sturdy and can hold at least 7 kg.

Why? Because you only want to buy a tripod one time in your life as a photographer and you will eventually buy longer (and heavier) lenses.

I use a Manfrotto with 3-way head for shooting stills and a Manfrotto 501 Fluid head for shooting video.

Your Nikon doesn't mind whatever tripod mounts beneath it. The best brand would be the most expensive. If you can't afford that, get the heaviest one your money can buy.