The best car alarm actually isn't a car alarm at all...

Allow me to explain:

Reviews of the Top 5 Best Car Alarms ("best car alarm" = anything that actually stops thieves, whether or not it is a manufactured "alarm" or some other type of antitheft device).

1. For those times you park on your own property, a medium-size or large dog can protect your vehicle and its contents, better than a manufactured car alarm, because besides making some noise, they come equipped with jaws, sharp teeth & a territorial, protective nature. Downside -- if a thief knows you have a dog & wants your car anyway, he may bribe, poison, tranquilize or otherwise take your dog out of action.
2. This "car alarm" is the best we've found for stopping car thieves. Called "Ravelco", it takes so much effort to beat that thieves are better off moving to the next vehicle. I researched antitheft devices & this one claims an undefeated track record since 1976. It even was undefeated when tested against a professional thief by insurance companies (CESVI), which no other antitheft device had ever lasted more than 3 minutes. It has been recommended by ABC, NBC & CBS News, Car and Driver Magazine, police officers and more. Downside is, it does nothing to protect the contents of your vehicle, but the contents of my vehicle are worth much less in both financial & convenience value than the vehicle itself. Besides, smash and grab theft happens so fast, neither a noisemaking alarm or nor 2-way paging alarms can protect your vehicle from a smash and grab thief.
3. Best "car alarm" for stopping a smash and grab thief: Flash fog. Click to watch the video. You're gonna love it. Expensive though.
4. Remote paging alarm with a Do-It-Yourself noisemaking alarm with extra powerful loud horns set just a hair or 2 past the threshold of pain, installed in the cab of the vehicle. (Just don't accidentally set it off yourself, or let your kids accidentally set it off... and beware of deaf thieves or car thieves with earplugs.)
5. If your main concern is tow trucks, then vehicle tracking by radio (like LoJack) or cellular signal may be your only chance to get the vehicle back. Know however that cellular & GPS jammers are available on eBay for under $100 "buy-it-now" sales . And even with "early warning", the police can't do anything until you file a report, so the thieves have plenty of time to take whatever they want & dump the rest of what used to be your vehicle. Or if they are amateur joy riders, they have plenty of time to wrap your truck or car around a tree or telephone pole for you. I searched Google news archives for vehicle theft by tow truck, and only found 2 cases in recent history: 1 was tow truck drivers taking semi-abandoned, junky vehicles to sell for scrap metal & the other 1 was owner fraud, where vehicle owners hired tow trucks to make their vehicles "disappear" to get a cash payout from their insurance company.