The point of a dom punishing a sub, is not to damage them out of anger, but both to train them to follow their commands and for the enjoyment of both parties. Different kinds of punishments can be effective for different kinds of subs, dynamics, and offenses committed. You always need to clarify to your sub why they're being punished (see grey area punishments below for more info). 'Most effective on' notes are flexible and vary depending on the sub.
⊹ Mental: Strike system/counters (the dom marks down a strike when the sub acts out with the intention of "cashing them in" later), writing lines, "timeouts" (can be modified to how it best suits your dynamic, such as the sub having to sit at the dom's feet rather than their lap, or balancing something on their head to keep them still, etc), extra chores around their house (bathroom cleaning, dishes, etc), mental bondage (making your sub stay in one place for x amount of time, moving as little as possible), privilege restriction (taking away masturbation/tv/treats/etc), lecturing, must ask permission for every little action (going to the bathroom, stepping away to do something, etc)
Most effective on: Littles, soft subs, slaves, domestic servitude subs
⊹ Humiliation/Degradation: Taking away the sub's ability to "act human" in lieu of acting like a cat/dog/other lower being, such as making them meow/bark/or using commands like sit/speak/down when talking to them, repeating degrading lines out loud per the dom's order ("repeat after me: i am a pathetic toy. i am only for daddy's/master's/etc's pleasure"), clothing removal, making them thank you after each spank/punishment, forced silence (not allowing the sub to speak whatsoever without your permission), body writing, forcing the sub to grovel/beg for mercy/to avoid punishments
Most effective on: Brats, degradees/humiliation sluts, masochists, pets, powersubs
⊹ Physical: Self spanking/paddling (tons of implements in the average house the dom can have the sub use), edging (making them masturbate and stop right at the edge as many times as you want, with the intention of eventually letting them cum), cold (ice cubes on nipples/thighs/genitals a limited amount of time, cold shower), overstimulation (turning a toy on too high, making them cum until they beg to stop, etc), having them pinch themselves (for example, nipple clamps) or slap their own genitals, ruined orgasms
Most effective on: Most subs enjoy at least one of the above, but especially effective on masochists/painwhores, brats, slaves, SAMs
⊹ Gray area punishments are punishments that can be extremely effective/enjoyable for some subs, and extremely detrimental to the well being of others. Talk to your sub before you include any of the following: Denying time with their dom, denying access to comfort items (stuffed animals, alcohol, etc), silent treatment/cold shoulder, public/riskplay, punishing for no reason (doing it for fun/because you own the sub and can do what you want/etc), orgasm denial (making your sub edge then stop completely, not allowing them to cum at all), forbidding sub to wear their collar, yelling at your sub
⊹ Other notes: Communication is #1, and the unwillingness to discuss alternative forms of punishment to better suit the situation/person is a red flag for both doms and subs. If your sub isn't okay with a punishment here, don't use it on them! Never ever use hard limits as punishments. Aftercare is always important, even if it's just checking to see if your sub is doing okay and they decline needing anything, and vise versa! Check on your dom's mental state as well and make sure they know you enjoyed it and appreciate them. Almost all of these can be doubled or tripled up - try one from each category simultaneously to multiply effectiveness by a lot! Stay safe, have fun, and keep it kinky.