The main thing I have to say is that for long-term replaying, by far the most valuable thing would be the facility to tackle any level with any desired weapon combo, or more generally, weapon set. I play a lot of H1 campaign (on Xbox), and although each level offers plenty of weapon types, I'm nevertheless limited. Just to give a few examples, I'll never have the pleasure of tackling level 1 with a shotgun, level 2 with a rocket launcher, level 3 with a pistol, or level 4 with a sniper rifle. Of course, with H1 they wanted to introduce weapons gradually, and maybe there were storyline issues to take account of too. You didn't get a rocket launcher until level 4, and a shotgun until level 6. That rational makes sense enough for your first few plays, but long-term it's a frustrating limitation. I wish I had complete freedom in H1, so I could enjoy using whatever weapon set I fancied. Think of all the extra fun you could have!

But how might such a thing be arranged in a shooter game? One idea that comes to mind is some sort of 'replay mode', which could perhaps be an unlockable. In that mode the player would be able to express an interest in a particular weapon set - perhaps a maximum of three favoured weapons say - and then the level would try to accommodate that with appropriate weapon/ammo availability throughout. This idea strikes me as probably being better than simply adding loads of weapons and ammo to a level, because in that case things could get too cluttered.

Now a few other things.

: Any weapon can be used to complete Legendary with skill and patience.

That's a bit generous I think. In order to be properly meaningful, let's say we're talking about killing most or all enemies (rather than simply bypassing them) and doing most of it with that weapon, and the rest with grenades and/or melee. In that case you'd have a seriously hard time tackling AOTCR using an AR, with its severely underpowered nature and horrendous scatter-shot bullet distribution. It's possible, and ok it does fall under your "with skill and patience" proviso, but we're really getting to extremes here, of a type which I suspect a lot of players wouldn't be willing to contemplate. The play would have a distinctly abnormal feel and you could expect to need a lot of boring backtracking for ammo. The needler would be even tougher, and is limited by being no good against vehicles.

: which games ammo and weapon availability do you enjoy most?

H1.

: Which do you think has led to the most replay value, and also the most satisfying combat?

H1 by a mile, especially in regard to "the most satisfying combat".

In regard to the other Halo games, one thing I often found annoying was a difficulty in using my favoured weapon throughout a level, due to low ammo availability. In H2 I loved the battle rifle but it was forever running out of ammo. In H3 I loved the Brute shot but that had the same problem; and when you're constantly concerned about running low, it kind of detracts from the fun (plus, in some situations your idiot Marines will advance into battle and get themselves killed while you're still busy looking for supplies). I seem to remember ODST as a particular offender for low ammo, although I didn't play that extensively so there may've been some ammo sources I never found. In Reach I often wanted more DMR ammo. Actually I didn't play that campaign extensively either, but I played Winter Contingency a lot and found DMR supplies scarce. In my limited time with H4 I likewise found DMR ammo annoyingly scarce. And thus…

: Do you enjoy being forced to use other weapons due to low ammo counts

No. It's annoying. Well, it can make for an interesting and challenging time of course, but generally I'm more interested in being able to keep using a favoured weapon.

By the way, talking of differences, I noticed that in H3 there didn't seem to be any ammo packs - unlike in H1. I forget how things went in the other sequels.