DAT can be anything, but in terms of video it is usually VCD which is MPEG-1.
MPEG-2 was designed to be capable of higher quality than MPEG-1, but that is only at higher bitrates.
VOB's generally mean MPEG-2 and generally pretty high quality since they are from a DVD. Still could be low or high quality.
MPEG4 could mean anything. Generally it refers to XviD video and mp3/AC3 audio in an AVI container.
As per above, AVI is just a container. Quality depends on the streams it contains, but AVI tends to have higher overhead than other formats, such as (from your title) mp4. Still it generally means XviD video and mp3/AC3 audio.
DivX is an ASP MPEG-4 video codec, still in terms of downloads it actually usually means XviD video and mp3/AC3 audio as per above.
Out of the above, taking MPEG4 to mean all things MPEG-4, the highest quality/size ratio would be AVC/H.264 video with AAC audio in an mp4 or mkv container.
Oh yeah, DivX is also a container. It is a hack/extension of AVI.
