Where can I get it?

Just search for it in your application manager.

There is no "best". It depends on your needs, your desires, your skills and your equipment.

For some people KMplayer, the front-end to mplayer for the command line works fine to display subtitles, for me it never did. For some people the unfinished version of VLC was good enough and there are plenty of cases where the stock media player does a fine job.

For most use cases it's simply a matter of preference. Only when you need more specific functionality, the non viable options weed itself out by lacking that functionality.

I use mplayer for command line a lot too on full blown desktops like netbooks. Kubuntu for Acer's eee netbooks, for instance, doesn't play well with the TV-out. With the command line it turns out to work just fine by altering the output channel for the video and directing it to the TV.

Specific: When we're talking about .mkv files, or flv's, or the need to alter the screen or audio output, my advice would be the command line version of mplayer, as the most versatile of all.

Lastly, some mplayer links:

Download page here:
http://maemo.org/downloads/product/Maemo5/mplayer/
Of course you can download it through your application manager, or it install it from command line, as root with the following two commands:
Find the manual here:
http://www.mplayerhq.hu/DOCS/man/en/mplayer.1.html

Wiki here:
http://en.wikipedia.org/wiki/MPlayer

But there is official version:
www.videolan.org/vlc/download-maemo.html
but how to install it?

If I click link, application manager doesnt load it.
If I goto directory there is .deb packages, but
application manager says cannot install.

teroyk,

Did you try download all the files from here and installing them via dpkg -i?

Yes it works this way:
Install from that link with dpkg first libvlc5 and then nox and then vlc-player.

No need to install the packages manually. HAM does load the repo, you just have to specify the dist as fremantle since that is left blank.

The real question is why you'd WANT to install it. There's no hardware acceleration and it uses ~100% CPU and can hardly play any content smoothly. Between KMPlayer and OMP there's not much need for another media player on the N900 these days.

install SiB player. Gui app for mplayer. its far better than vlc player for N900.