DEI Sidewinder 7000 alarm. Has most every features (two-stage sensor, engine cut-off, fuel cut-off, valet, and the brain senses any electrical stuff going on like the hood/trunk/dome light that will trigger it). Saved my car the one time it needed it. My stang was in my driveway and someone tried to use a coat hanger to get in the driver door, but as soon as they unlocked the door the alarm went off immediately. I ran to the window and they person was gone. My driver's side door was open, but not one item was missing. I figure I got a peak at my car within 10 seconds of hearing it. I took a look around and reset the alarm again. Nothing since.
As more mods come into play, I'm going to have a local "car alarm" shop update the alarm's brain and reconfigure the remotes to better suit my needs. A couple features need to be added, but mostly I'm looking forward to getting a siren that literally causes pain to be within a few feet of it (will take a while to get my hands on that). My alarm rarely ever goes off, so I'm not concerned about annoying others. A good thunderstorm can set it off though. Thankfully, my car spends most of it's time at work or in my garage, so it's not out in plain view most of the time.
All my future upgrades don't involve cosmetic stuff, so that's probably the best deterent of all. Basically, my car is 10 years old and looks completely stock. Fortunately, it runs great and is ready for some upgrades. 1994 GT's didn't come with any type of "theft deterent" system (at least mine didn't), so something absolutely has to be added.