Whats the best methods of hiding a nuke attack from the opponent?
A couple I can think of:
- Do double nukes. If they kill one ghost, they may not even realize there's yet another nuke headed their way.
- Nuke right after you begin engaging their army (so, while you're in combat). They may be at a loss at how to deal with it. Finding a nuke dot takes precious time away from your micro, so try to launch the nuke at amoment when it's important for them to micro.
- Put the red dot in un-obvious places. Right in the middle of the mineral line is a bit obvious, but maybe put the nuke center on high ground, or on a vespene geyser (for Terran the refinery even has a little red dot built into the graphics). People look for the dot in the center of things - I always try to put the dot at the base of a building, right where it meets the ground.
As you have already discovered, using a nuke against enemy units is very difficult since a player is very likely to notice it. The trick is to only use it to harass enemy units when your opponent is otherwise distracted. When you do use it, it is best to use it against an area on the map he is not currently focused on. If your enemy is attacking you, launch a nuke on one of his bases (a successful nuke will deal massive damage to any of the bases, and destroy any workers as well).
However, I believe you may be narrowing your focus too much. The nuke is useful against more than just units, it is devastating to buildings as well. The following buildings will be destroyed from one nuke (source):
Pylon, Supply Depot, Extractor, Refinery, Tech Lab, Reactor, Photon Cannon, Bunker, Missile Turret, Sensor Tower, Spine Crawler, Spore Crawler, Creep Tumor
Of these, I would say the supply depot is the most tempting targets. Terran players will often clump up their supply depots together. One well placed nuke can put a huge dent in the enemies supply cap.
Finally, the defensive nuke. The defensive nuke will almost always be seen by the enemy, but that's fine. One of the most useful features of the defensive nuke is that it can hold off an opposing army, either giving your units time to retreat or finish building some units. Managing to kill enemy units with it would be ideal, but considering you need to place it in the opposing armies path for it to work, it's very unlikely your opponent won't notice it when moving his army.
The other answers have already given good advice, but I wanted to add a little that I've been successful with:
Multiple nukes, attacking: as simultaneously as you can manage to launch them (two or even three).
- Same area: make sure that your ghosts are on opposite sides, so the opposing force won't find and kill them both if they start searching.
- Different areas: usually though, I like to nuke different target areas (often the main + [2nd] expansion base), so they are unlikely to find both ghosts when searching (and may not even realize there are two, if you get the timing of the launches down fast enough).
Multiple nukes, defensive: with a ghost parked somewhere far from your base in relative safety, when attacked, you can nuke their retreat simultaneously with their advance (with a ghost from inside your base). This is often very effective at devastating their army. I also find this is a great time to drop one onto their base, assuming you've got another ghost out there on the map somewhere.
The edge of a cliff. As Wipqozn points out, nukes are excellent against buildings. I often like to drop a nuke on the edge of a cliff that has defensive buildings perched atop of it. Even without sight, they get damaged if not destroyed.