I was reading the internet & try to look for the best alarm for the highlander. But it's a tough search...
Here are the criteria for the car alarm:
#1. No monthly subscription
#2. One-time payment only in buying the alarm.

Here are the tips I found:

1. "If you're set on buying the best car alarm available because you want something that will signal you immediately upon a break-in, LoJack's Early Warning System is the way to go. The best car alarm money can buy will only make noise to deter a thief. But LoJack's Early Warning System will actually alert you—by phone, pager or email—the moment your car is being moved without permission."
http://www.caralarmsystems.net/best.htm

2. "Higher end models come with microphone sensors which detect breaking windows and other sounds.
More expensive alarms are starter alarms. These car alarms allows 2-way communication between your car a handheld monitoring device, letting you know if you car has been started and allowing you to disable it at the click of a button. "
http://www.galttech.com/research/cars/best-car-alarms.php

3. " * Make sure the alarm shuts itself off after two or three minutes.
* Look for a system that has a starter disabler.
* See whether the system will arm itself even if there is
a faulty switch.
* Make sure there is both hood and trunk protection.
* Look for a system that allows you to silence the "chirp" that occurs when you arm or disarm the system.
* Try to get a system that will ignore a false-alarm trigger by bypassing a sensor that sets the system off repeatedly during a certain time period.
* Look for a system whose shock sensor can be adjusted—an "adjustable dual-stage shock sensor" is good to have--so your alarm won’t go off every time a squirrel jumps on your car or a teenager leans against it.
* Check whether you system will allow you to add certain features, such as a keyless trunk release.

Check whether the system includes an "anti-scan" capability, which will stop thieves from trying endless
numbers of codes to try to bypass your security code."
http://www.caralarmsystems.net/buying.htm

4. "the better car alarms will also be equipped with sensors on the windows as well, just in case the thief bypasses the doors and breaks the window to gain entry instead. This allows the thief to help himself to any valuables left in the car or, alternatively, haul the vehicle away with a tow truck. Even the motion sensors and temperature sensors that form a part of the best car alarms, are not guaranteed to stop the thief who uses a towing vehicle to remove your car....
The absolute best car alarms will also have tilt sensing switches to recognize when the vehicle is being lifted by a tow truck, or even when it is moved using wheel sensors. "
http://searchwarp.com/swa303459.htm
5. "Feature#1
Active Re-arm: to prevent accidental disarming from the remote transmitter, if the alarm has been disarmed the system will automatically re-arm after a specified amount of time if a door is not opened.

Feature#2
Air Horns: are used to call more attention and cause panic to the intruder if he makes it inside the car.

Feature#3
Anti-Code Grabbing: a typical method employed to change the alarm code with every arm/disarm that ensures it to protect from the code grabber.

Feature#4
Carjack Protection: will cause the engine to turn off and the siren to blare, incase the vehicle is taken forcefully from the owner.

Feature#5
Emergency Disarm Switch: hidden and is used to disarm the alarm in case of an emergency.

Feature#6
Ignition Kill: will disconnect the ignition if the alarm is armed.

feature#7
Microwave Detectors: mostly popular with open vehicles these are proximity sensors that trigger when an object enters the sensors zone of protection. "
http://www.hidstores.com/blogs/2009/07/upgrade-your-vehicle-safety-with-alarm-sy- stems/ (http://www.hidstores.com/blogs/2009/07/upgrade-your-vehicle-safety-with-alarm-systems/)

With All this tips....

What alarm system do you recommend to be the best for highlander with no monthly payment?