Short answer: Federal 8 pellet 00 Buck. It's sometimes sold as "law enforcement" or "reduced recoil" or whatever but I'd start there, make sure it works in your gun, patterns well, and if you have a problem with it branch out from there.
Long answer:
You don't want to be using #2 or #4 for defense. Penetration is going to be pretty crappy. Rough guess, maybe 8" into gel on #2. It's one of those things where I'm sure I've looked at a test, said it was crap, but didn't commit the final number to memory because I don't bother remembering just how ineffective some rounds are. I just put them in the ineffective group.
And you can be sure they're not talking about #2 or #4 buck (which I would say are viable, but not popular) because the pellet:weight ratio can't get you a 1.25oz load. There's 15 per ounce in #2 and 21 in #4. they don't divide evenly. You can do it with 00 Buck, but you'd end up with a really weird stack in the shell with 10 pellets and you'd be in the 3" size by then.
Anyway.... 1.25oz at 1250fps is going to thump heavily on the sending end. I do not like high recoil shotgun rounds. Hell I don't even like 1 1/8oz trap loads at 1200fps. They annoy me with the recoil and you'd be hard pressed to find a trap shooter that honestly thinks that heavy payload (vs 1oz) does them any good when they're working off the 16 yard line. But the 1 1/8oz shells are what sells as the cheap fodder because if you put them on the shelf next to one with only 1oz of shot most people are going to buy the one with more pellets because they think it helps.
It doesn't.
Now, that's trap/skeet/sporting clays talk but it relays into the HD world too. The increased recoil, when compared to something like Federal's 8 pellet 00 Buck load, gets you less penetration, but more holes. Is that worth anything? I don't know. Let's survey the guys shot with 00 Buck and ask them if it worked.
*crickets*
Yeah, you don't ever hear about a guy hit with 00 Buck and doing much after that. You get it with 7.5# shot stories, where sometime they're DRT, or they continue the fight, or they get fixed up in the hospital, but stories about shrugging off 00 Buck haven't come across my plate.
The nice thing about Federal's 8 pellet 00 Buck loads is they put you in that nice docile region for the shooter and in the "bad idea" camp for the target. A shell with 8 00 pellets has a payload of 1oz at 1145fps which is quite comfy for anybody used to shooting a 12 gauge. Not comfortable on the other end though.