If you are asking for quality, non of them, cause they are just multimedia container,

Currently the best video codec is H264, and the opensource implementation, x264.
and its competition codec, VP8, Ogg Theora, Dirac, FFmpeg Snow, and loads more which many info you can find on wikipedia yourself.

MKV is not a video format, but a multimedia container format,
read here: http://en.wikipedia.org/wiki/Mkv
Yes MKV is the best... the best container.
The best video format currently is H264 and very soon, h265.

why MKV is better?
it support all the audio/video codecs and subtitles formats, that rmvb/avi supports.

AVI is ancient container which cant support audio with VBR without hack, normally you will get audio/video out-of-sycn if mux VBR mp3/aac/vorbis/...(endless of list) eith conventional method.

Also, it cant support H.264 specialized B-FRAMES, Dxva/hardware acceleration does not work if your H264 in inside AVI.

while RMVB is exclusive for Real network's audio/video formant only.
they are outdated and no longer updated IIRC. so quality is far behind h264 already.

LOL, dont know how to reply to this post
read the previous post by other member please, im pretty sure you know how to read

The development of RMVB codecs a.k.a. RV40 stopped long time ago, basically the quality is much inferior compare to its competitor aka h264.

now their development only focus on the multiplatform video player.
http://en.wikipedia.org/wiki/Helix_(multimedia_project)

you are ignoring the video resolution and the quality here.
Most recent video codec also could compress a 45mins video to 300mb size but may be with poor video quality and low video resolution.
perhaps h264 or vc1 could do better.

Absolutely Not right from your reply
720/1080p is video resolution, not video quality, 1080p means 1920 or 1440 x 1080 video resolution in Progressive Mode.
And read more wiki... MKV, AVI and RMVB ARE MULTIMEDIA CONTAINER
DVD are MPEG2 video with up to ~9Mbps bitrate, VCD are MPEG1 video 1.1Mbp/s.
AVI and RMVB quality can be vary because the encoder can encode them using various profile + video codec, You cant compare them with VCD or DVD, its like compare orange and apple

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Added on January 23, 2011, 1:57 amDont worry, normal ppl dont even know what the heck is AVI/RMVB/MKV. Codec stuff is for geek only lol.
i think its quite fun when ppl keep say MKV are better, then i will keep reply the same stuff, i must be crazy or nothing better to do LOL

This post has been edited by 1024kbps: Jan 23 2011, 01:57 AM

AVI is still as good as mkv if i understand this page http://www.alexander-noe.com/video/amg/en_myths.html

But it does not support B-frames, and h264 make thing worst because h264 use arbitrary b frame order.
Plus DXVA may not work if your H264 was muxed into the AVI container, your PC may crash or the video player will simply give you "Green Screen of Death"

Dont associate H264 with MKV, its great on everywhere regardless the type of container.
and you are discussing something off topic.

Video dimension and compression format must taken into consideration too,
Large file size not necessary mean its better quality, unless you are comparing 2 video with same dimension, playback duration and compression format.

For example if i uploaded a uncompressed RGB video(hundreds GiG) to a net, again i uploaded a h264 compressed video(10GB), both are from same source(MPEG2) btw,
which one will you download?

Its same for Image and Audio too.

This post has been edited by 1024kbps: Mar 21 2011, 06:35 PM