So you've got an actual copy of the Extended Edition? I don't know if it's worth much now, seeing as it's available in the 30th Anniversary Commerative Set (limited to 20,000 copies), but it was out of print for the longest time.
If you want to stick to the Michael Myers films then I'd say stay away from Halloween III as it has nothing to do with him. It is a personal favorite of mine though, and is actually a very good movie. I'd recommend the 25th Anniversary DVD, if you can find it, of the first film. Halloween II has only been released once and it's a bare bones release. Halloween 4 and 5 are both available from Anchor Bay's DiviMax line with some decent extras. Halloween: The Curse Of Michael Myers, aka Halloween 6, only has a bare bones release. Halloween H20 only has one release as well, and aside from a couple of extras it's pretty bare bones. Halloween Resurrection is definitely the worst film in the franchise, but the DVD has a very nice selection of extras.
If you want to go the Blu-Ray route though I'd recommend the Anchor Bay Blu-Ray of the first film, it's the only one available and can be had for 10$ at HMV. Halloween II has been released by Universal and Shout!. The Universal disc contains the alternate scenes shot for the tv cut of Halloween II, an alternate ending, and a "documentary" called "Terror In The Aisles", which up until this Universal disc had only been available on VHS.
The Shout! release, which came out this Tuesday, contains a better transfer, two audio commentary tracks, a 45-minute long documentary on the making of the film, an "episode" of Horror's Hallowed Grounds, a photo gallery, the alternate scenes shot for the tv version, the alternate ending, the film's trailer, tv spots, and radio spots. The second disc is a DVD and contains the film's alternate tv version along with the film's screenplay in .pdf format on the DVD-Rom side.
Halloween III was also released by Shout! this Tuesday and contains a nice set of extras as well. But if you're sticking to the Michael Myers films you might want to pass on this one, although I'd definitely recommend checking it out.
Halloween 4 and 5 were released on Blu-Ray last month by Anchor Bay. The extras however are the exact same ones that were included on the DVD releases so there's no real reason to upgrade other than having the films in 1080p.
Stay away from the Blu-Ray releases of Halloween: The Curse Of Michael Myers, Halloween H20, and Halloween Resurrection. Both the Canadian release featuring all three films on one disc and the US releases from Echo Bridge Entertainment are pure crap. Stick to the DVD's for those.
If I were recommending the films though I'd say go for Halloween, Halloween II, Halloween: The Curse Of Michael Myers, and Halloween H20. Halloween 4 and 5 aren't bad, just not my favorites.
And if you need any more convincing regarding the first two films let me just say this: I've already watched Halloween and Halloween II four times this year. And the last two times were today and yesterday.
P.S. Give Halloween III a shot even though it has nothing to do with Michael Myers or the rest of the franchise. It's a pretty decent film in its own right.