There really isn't much of a set time for when lines for mazes start to form, but starting to scope things out around 5:30 (or early) is probably a good idea to get a jump on the crowds.
I don't think any one maze particularly stands out above the others, but the most consistent way of comparing them is their level of detail. Haunt's haunted houses are known for their extreme attention to detail; each maze is a permanent, year-round fixture with impeccable levels of storytelling and immersion. To this end, I think Condemned and Blood on the Bayou are the best (in part because they're the newest), but Condemned has an extra gimmick where the maze is almost entirely in the dark, and guests are given flashlights to navigate the maze and see the details.
However, the biggest factor in what makes a maze successful is its cast, and it's impossible to know which mazes will have the best team. Even Blackout, a maze literally shrouded completely in pitch blackness, can be amazing when its cast is strong. A smaller of weaker cast, however, means your best bet for enjoying a house is to take in the scenery. EDIT: What Zachary said.
Also, I forgot to mention it earlier but one of Haunt's signatures is a nightly opening performance at the Grand Bandstand near the Eiffel Tower, which starts at 7. There's a new show this year, but it should follow the same general format as in the past, wherein all the park's scare zone monsters are "summoned" and then "unleashed." It's always an incredibly cool spectacle, and the energy from the show and the crowd is unbelievable. In the past, this has consistently been my favorite part of Haunt, so don't miss it.