I’m not sure any starting hand in Hold’em inspires more debate than ace-king. The debate usually centers around how the hand should be played pre-flop in tournaments; one player might advocate a flat-call in a situation that another player would five-bet in. How aggressive you should play ace-king will depend on stack sizes, position, your opponent, and your image.
Everyone knows to open ace-king regardless of your stack size or position. The difficult situations are knowing when to get aggressive with ace-king pre-flop, and when to just call a raise (or three-bet) made in front of you. Although position plays a large role, it’s easiest to discuss these scenarios by effective stack sizes.
When you’re in a situation with 20 effective big-blinds or less, ace-king is (essentially) always a hand to get all-in with pre-flop; regardless of position. I guess maybe if the tightest player I’d ever seen open raised under-the-gun with a twenty blind stack and I was next to act with ace-king then I might flat call…but baring something that extreme, you can be comfortable getting all-in with ace-king at this stack size.
When you’re working with effective stack sizes between 25 and 45 big-blinds, you’ll often be re-raising ace-king pre-flop with the intention of getting all-in, but there’s more room for discretion. In some cases, you may be facing an opponent where you think ace-king plays better as a flat call—such as an opponent that’s tight pre-flop and weak post-flop who raises in early position. There can also be cases where ace-king presents an opportunity to trap players behind you: if you’re sitting on 40 blinds and facing an early position raise with ace-king, it may be better to call if there are shorter stacks behind that could go all-in (sometimes you should call if the open raiser goes all-in, and sometimes you should fold…knowing when will be player dependent).
When you’re playing ace-king with effective stack sizes between 50 and 80 blinds, you’ll still be re-raising in most situations, but how aggressive you can be becomes very position and player dependent. For example, if you raise a 60 blind stack on the button and a capable player in the small blind three-bets you, it’s likely correct to four-bet with the intention of getting all-in. But if you raise a 60 blind stack in early position and a capable player behind you three-bets, it’s likely best to just call the raise and play out the hand post-flop.
Ace-king is still a re-raising hand early in the tournament—when average stacks are 100 blinds are more—but its unlikely a good idea to get all-in pre-flop. Of course, there might be some wild players who vastly overvalue inferior hands, but in most cases, you should re-raise ace-king against an open with the intention of just calling if your opponent four-bets (and maybe folding if they’re tight and make a huge four bet).