Honestly, a good primer is Terror Tram. It's HHNH's most unique attraction, is awesome to step foot on the backlot, not too intense but gets your body warmed up for everything else, has photo opps, and ofcourse you get to see famous movie locations.
It was my very first attraction when I went in 2007. I literally went JUST to do it and I was all by myself and it was the very last day, but I didn't want to miss it for the world. Funny enough, I had walked around the backlot before for a random 5K, but this was conjoined with Halloween spookiness and my all time favorites (Leatherface, Jason, and Freddy), but it still felt like I NEEDED to be there. I didn't get into the park until 10pm with the park set to close at 12am, and I knew I'd only have time for that and one maze. The most buzzed about maze (pun intended) was Texas Chainsaw Massacre: Back in Business, which is still to this day the best maze I have ever been through. Luckily, they extended hours to 1am and I was also able to get through Friday the 13th and Nightmare on Elm Street, plus see the last showing of Bill and Ted. Crazy how that one extra hour made it able for me to hit all the mazes and the show. But even if I didn't, I honestly thought the Terror Tram and that one maze would've been worth it.
Sorry for going down memory lane a bit, but yeah, Terror Tram is my vote. Followed by Killer Klownz. And oddly enough, even though the Weeknd is in a soundstage, that venue suffers from insane light bleed during the day, so if you can hold it off until the sun has gone down a bit, I'd recommend that.