It's reaaaaaaally hard to pick just one thing. When it comes to play I'll take anything and everything (within our limits of course)
When it comes to actual discipline... There's almost like a perfect trifecta, but it's not always the same combination. Although now that I'm thinking about it I guess corner time of some form or another is probably used most often. But it could be some combination of corner time, spanking, mouth soaping, writing lines... Pretty much all the common stuff. Most of our prescribed punishments that I get used on me are pretty typical corporal punishment stuff. The punishment needs to be a journey most of the time for me lol. I mean for small infractions, a quick spanking or a 5 minute time out is fine. But, most of the behavior I struggle with is not just "small" things.
I like anything that has a variation or twist on it, or some sort of "if you don't accomplish this, you'll have to take this additional penalty" because when it comes to actually being punished for real, I very much want to get out of it everything I can and let it be constructive. I'm already learning discipline by being disciplined in some form or fashion, and of course I'm learning my lesson as to why I'm being punished in the first place. But if I can also learn some focus along side it too I really appreciate that. So some examples would be like:
Instead of just "corner time", sometimes we use "wall time" which usually involves me being made to hold some sort of object against the wall with a part of my body. If I drop the object I face additional punishment and/or have to start over. Another alternative is sometimes my girlfriend will "put me in the closet." We have a closet in a spare bedroom that is empty. She does not lock me in for safety reasons and because then we're spilling over into sexual/play territory. But after a certain amount of time she comes and gets me, and the expectation was that I held a specific position in the closet during my time in there. This one is interesting because it forces me to be honest. She isn't supervising, so if I fucked it up I'm forced to tell on myself. Again, another learning opportunity that may carry additional consequences.
Instead of writing lines, sometimes I have to write an apology letter or essay detailing my punishment to her (before or after). In any case sometimes I have a time limit set for me. There is almost always a word count, and again if I don't meet these criteria than I face additional punishment and/or have to start over.
Sometimes with a spanking if I don't remain perfectly still in position, or stay on my tippy-toes, or keep my ass stuck way out, there are additional penalties. You get the idea at this point.
My girlfriend is very good at finding these sort of focus anchors for my punishments, which I suuuuper appreciate and love. It also leaves something to the imagination so there's still a fresh element of fear and anticipation when she tells me "Okay, you just earned yourself a punishment" which is a major element of the effectiveness of the whole thing for me. I need to have the nervous butterflies in my tummy to an extent, otherwise it just doesn't have the same effect. Of course everything is consensual and agreed upon. I know she's never going to throw me a massive curve ball we haven't discussed in the past. Sometimes these little twists are easy, sometimes they can be quite challenging (a 3-5 minute corner time isn't anything to scoff at when you have to stand on your tip toes the whole time, knowing you are going to have 40 stripes on your ass from a cane if you don't stay like that). Sometimes they're just challenging because of my emotional disposition or headspace about whatever I did wrong is. Most importantly though they are always proportional. We don't stick to most people's definition of "let the punishment fit the crime", because in our case for example, mouth soaping is used for more than just me swearing. Like I got my mouth washed out a few weeks ago for breaking a bowl because I was getting too frustrated emptying the dishwasher and was going too fast. I know for a lot of people that doesn't make sense, but for me it does. It helps me stay on my toes and keep my focus a bit better. It gives me that extra push of encouragement to make the right choice. That way I can't go "Oh, well if I accidentally break something it won't be so bad, because I'll probably just get a quick hairbrush spanking. That isn't so bad, I can handle that."