In my opinion, a DVD should have the definitive version of the film in the best quality with the most informative extras. I've compiled many, many custom discs for myself over the years (R4 Australia got shafted a lot in the early days). I would love to keep this discussion going to find some titles and things I am as yet unaware of being available.
I'll give some info on some titles now, will do some more at a later date. Here's some titles worth looking at the R4 Australian release for and why. Note, I don't have all of these, but I work in the DVD department of an entertainment store and through that and my own research online I know a lot...
TMNT (2007)
The DVD-retailer exclusive 2-disc set has the same extras as the 2-disc set elsewhere in the world, however the R4 AU is the only one that doesn't force the commentary on you. You are able to listen to the original audio.
Timecop (1994)
Magna Pacific's R4 AU release is the only one I'm aware of that has DTS sound and the original 2.35:1 ratio, anamorphically enhanced of course.
Shriek If You Know What I Did Last Friday The 13th (2000)
A commentary track not found elsewhere and is anamorphic.
Prom Night III (1989)
Uncut version of the film. The UK/US releases are the "tv edit" version which removes I think 5 minutes of stuff.
Mr. Holland's Opus (1995)
Original 2.35:1 ratio, anamorphic, 5.1 DD sound.
Heaven's Gate (1980)
Same extended cut as US DVD but it's anamorphic.
3000 Miles To Graceland (2001)
Has a bonus 11 minute featurette.
Transformers: Beast Wars - Seasons 1-3 (1996-1999)
Doesn't have 5.1 sound like the US releases, but we do get tons of exclusive extras: commentaries, animatics, galleries, etc. Madman pulled out all the stops on these sets. Love 'em.
Lilo & Stitch (2002)
We had the best release for the longest time. 2-disc Special Edition. Uncut, DTS, audio commentary and the 2-hr documentary with footnotes. I believe this has finally been released in R1 US though. The R1, original release at least, had a "Young Voices Of Hawaii" featurette not on any other release. Whereas the R2 UK includes a music video "Suspicious Minds" by Gareth Gates.
I'll get in now and add that I use several sites to compare picture quality, differing cuts, sets of extras and general R1 vs R2 vs R4 etc. The major one to reveal for those that don't already know it: Rewind's DVD Compare. Godsend for this stuff.