i like how open-ended this question is because homestuck likes making it impossible to truly pin down what an "act" is

someone could say act 1 is their favorite and someone could say act 6 is their favorite and they're both correct even though act 1 and act 6 are a legitimately impossible comparison. that's downright beautiful

anyway my answer to this question changes based on the day, the weather, and the position of mercury, but right now i feel like showing some love to Intermission 1, believe it or not, and i'm counting that as an act

i think it's extremely important to homestuck's structure. like you're following these kids, and there's been interjections with the exiles, but these storylines are clearly linked. and then here comes this giant interjection of a story full of characters you just barely recognize from their earlier cameo, and it's like whoa buster we're doing something new now

it's a good crash course on the kind of intricately lacing time travel bullshit homestuck adores. it's a great palette cleanser from the story up to that point with its more sleuthlike presentation. it sets up things that are important later that feel like an incredible payoff because you recognize that! that's the stupid bullshit from the intermission

also when it comes to homestuck, generally (and especially in the future from that point) being cool with other storylines taking the stage for a bit and having faith that it'll all be important is a good skill to be primed on, which is also what the intermission does. keep in mind hussie was setting up for hivebent by this point, in a world where no one ever heard the phrase "skip to act 5 that's when the trolls come in" yet. it's so interesting to read the intermission knowing that. like that whole thing really was hussie making sure to cover their bases and sow some goddamn seeds. feed us our medicine wrapped in a tasty slice of cheese so we don't notice. hell yeah