I wouldn't fit anything other than Clifford, I have been using them since 1996 and have never needed to look at anything else. Their alarms are circuit protected, this means that should someone smash your rear light and the parking light bulb inside they still won't be able to get into your car. Since many alarms also flash the parking lights when triggered, the short circuiting of the parking light bulb would lead to the short circuiting of the alarm as well, making it short work for the thief!
They also include a feature called passive arming. This is the system's ability to arm itself automatically. When placed in this mode, the security system will arm itself in 30 seconds after the you leave the car. Even if you forget to arm the system, passive arming will protect the vehicle. This feature can be used in conjunction with power door locks and windows to fully secure the car.
Alarm wise, Concept 650 MKII.
You want a good shock/impact sensor. Besides sounding the alarm, if someone opens a door or your boot you want the system to be able to detect a sharp impact, which is the job of the shock sensor. You'll want a dual stage shock sensor which discriminates between a light impact (which provokes high pitch chirps) and a heavy impact (which triggers the full wrath of the siren).
As mentioned above the window closure is a nice touch. An audio sensor to detect breaking glass, radar sensor, tilt/motion sensor to detect jacking if someone want to try and lift your car on to a transporter or tow the car away.
HTH