I've gone every year since 2006. It's hard to order them exactly, but I'll try.
1. HHN 18: Reflections of Fear - This year was a great mix of scarezones and houses. The scarezones were all top-notched, with Path of the Wicked and Asylum in Wonderland leading the way. It also had some of my favorite houses ever, with Dead Exposure and Scary Tales 3. Also, it was the most immersive event I've been to, with Bloody Mary's story present throughout (and Bloody Mary appearing in all of the houses).
2. HHN XVII: Carnival of Carnage - My second year, but the first time that I actually followed all the hype surrounding it with the website puzzles and whatnot. The scarezones were lacking this year (there was not supposed to be any safe zones throughout the park, but that didn't really happen) which is why it is not my top year. The houses though were some of my favorites. Dead Silence was amazing and is possibly my favorite house ever, and Psychoscareapy: Home for the Holidays seemed so unique to me at the time, it was just so fun to go through.
3. HHN XX: Twenty Years of Fear - All of the soundstage houses were amazing and actually pretty scary. Both tents were consistently good, a first for me (I always feel that one tent is a little inferior than the other), and the other houses were really detailed. The scarezones were awesome and I liked getting a history lesson throughout the event.
4. HHN XIX: Ripped from the Silver Screen - I really liked this year which is why I find it hard to rank it so low. This event was really immersive with the whole movie theme. The Usher was a great icon with a cool story and I loved his Silver Screams house. The soundstage houses were all very detailed (especially the Wolfman) and the other houses were pretty cool too (with Leave it to Cleaver in my top five favorite houses ever). The scarezones were all ok, not my favorite year for the streets though.
5. HHN Sweet 16 - My first year. I actually only made it through two houses because I didn't know what to expect and wasn't ready to wait in the long lines, but it did begin my love of all things Psychoscareapy. The scarezones were all pretty good (Harvest of the Souls would probably be my favorite scarezone ever) and the Arrival show was great. Finally this was my favorite Bill and Ted show. The only reason this ranks last is because of only getting through the two houses.