26" but a gun that fits and you have cofidence in is great at any length.
Plus the obvious differences between O/U, Semi and pump
PS: Just saw your Skeet post. I recently bought a shotgun that I intend to use as "double duty" HD, Clays, 3 gun and bad weather bird hunting (Ok that is 4 uses). It happens to be a 26" Benelli M2 American in 20 gauge.
I am going to let my GF have her favorite Beretta of mine and then the Benelli back fills it for me but also gives her an HD/gaming shotgun that uses same basic controls as her favorite clays gun. I am just going to pick up a second barrel for HD use. It is not a critical gun as she has Handguns, AR's and a PCR but the shotgun is a good multipurpose beater.
the pros have started switching to 30inch barrels. they like the swing better. i wouldnt go any less than 28 that way i could have a gun i can shot other clay sports with. and you would be hard pressed to find a quailty gun for skeet under 28in barrels any way. there are field grade gun with short barrels. but if it says skeet on the side it no less that 28.
You can find Browning 26 inch skeet as well as Remington 3200s, Berettas, SKBs, etc. I generally shot 26 inch for skeet for many years, I also used the same guns for upland game. I would just start going to a skeet field and talk to folks. Even if they won't let you shoot their gun most will let you try it for fit. Get the feel for as many as you can and then start looking from there. Jaqua's in Ohio will even let you shoot some birds if you are looking at a used gun.
Shorter barrels swing "faster", longer barrels swing smoother (help you continue that swing through after pulling the trigger). Real skeet (regulation field) is a short range game. I have always used 28" or 30", my personal preference.
I went on a skeet shooting tear when I first moved to Indiana and my favorite gun was a 28" barreled 391. But I never shot sporting clays worth a dime with it, preferring a 30" version of the same.
As mentioned above, what ever fits you best is the right length for you. I have different brands with different barrel lengths because in thet model X barrellength worked best for me.
I went on a skeet shooting tear when I first moved to Indiana and my favorite gun was a 28" barreled 391. But I never shot sporting clays worth a dime with it, preferring a 30" version of the same.