For most guests to Disney's Hollywood Studios, the Tower of Terror is a must-do attraction to cap any trip. After all, Tower of Terror was recently voted the "best attraction in America" by readers of Theme Park Insider. So with all of this popularity, it's going to be hard to figure out the perfect time to ride Tower of Terror but it can be done with a lower wait than you might imagine.
General recommendations: Tower of Terror has an advantage that most super-headliner attractions at Walt Disney World don't enjoy and that's the ability to tolerate a large influx of guests. All things being equal, Tower of Terror will likely have the lowest wait times at any given time of the day compared to Rock 'n Roller Coaster or Toy Story Midway Mania. This fact helps in giving greater flexibility to when one can ride. Fastpass tickets are usually available well into the late afternoon and evening even during the busier summer months but you should be assured of being able to snag one until the very least, lunch time.
Perfect time to ride:
Park opening - 10am
When the Studios first open for the day, the wait is zero minutes and even the large swarm of people who go to ride once the park opens up is quickly absorbed into the ride. If you play your cards right and don't waste too much time (or get stuck somewhere along the way for whatever reason), you can work in two to three rides in that first hour the park is open.
The Block Party Bash parade draws away a great number of guests away from attractions and because the parade is basically a "moving show", it lasts longer than most conventional parades. An added bonus is the parade in effect quarantines off Sunset Boulevard from the rest of the park, crowds should be more manageable.
During a showing of Fantasmic!
This can be somewhat tricky to time, but once Fantasmic! actually starts, it's a great time to ride Tower of Terror since many guests in the park have been lured to the show and often if the show is occuring during park hours, you can see a drop in crowds at Tower of Terror.
Times to avoid:
Most guests filter into the Studios in late morning or so and many more will stick around until about mid-afternoon before leaving the park for a break or simply because they've had their fill of the Studios. The wait times for Tower of Teror will likely crest around this time and you should avoid the area in general during these hours.
The evening crowds build up on Sunset Boulevard because of the dining options on Sunset Boulevard as well as to get in the area for Fantasmic!. Many guests will try to work in "one more ride" before Fantasmic! or departing for the day and crowds can rebound from their late afternoon lulls. Avoid Sunset Boulevard in general until Fantasmic! actually begins and the last people who were waiting for the show have gone to see Fantasmic!.
Insider's Tip: Ever gotten to the boiler room and the queue to go to the left or ahead are both congested? Go with the line ahead because the distance that line has to go is the shorter of the two so you'll likely be loaded on to an elevator quicker.