First, you shouldn't base your build order, on getting a quick killer drop, because your opponent can tell by seeing the early assimilator, and cybernetics. Second you need a robotics which is later in the game, making you have to play this strategy as an option depending how the game progresses. Third it's effective if the land path to the target is long or hazardous, making it a good choice. Fourth,..... well you gotta pull it off. Also it helps to have island/plateau bases in the game, making the shuttle worth getting to expand.
Ok now, you have a wide variety of units to place inside of shuttle. Anything fits, just that some are bigger than others. I'm sure sticking in 4 zealots won't do as much damage, as 2 zealots and a reaver. So mixing it up is a good idea when you are preparing to drop. Early in the game, the speed upgrade isn't needed, since defenses won't be fully setup. However you should have it, because it's necessary to ensure that the shuttle lives. It's a huge lost if you were to lose the shuttle, so try not to let your opponent see it coming, otherwise you might meet a nasty surprise at his base.
The whole purpose of the drop is to clean out all the workers and destroy his economy. This of course, should at least give you a huge advantage, since he'll be put on the defensive for a long time. So you must include a unit that destroy another unit fast and in large numbers. A reaver is a good choice since, you already have the robotics, and all you need is a support bay. Templars are also good, but require a lot of gas and may be a late game option. Archons to some extent are also good. Now you must realize that the cooldown of dropping a unit from a shuttle prevents it from firing right away. This makes unloading and loading an old tactic. So you have 2 choices with this, one drop and sacrfice the units, or drop for a 1-2 shot wonder and run away. Use whatever you feel will kill the most workers with the defense that your opponent has.
It's good to know that a reaver can build scarabs while in a shuttle, so as soon as your last reaver is ready, quickly produce the scarabs, and place it in the shuttle. You should always come from the back of the base, making it less likely you'll be seen, and hit the workers faster. Flying through anti-air defense is a good idea, if you have the speed upgrade
