I've done a ridiculous amount of research on this topic. If you cruise around radardetector.net forums, the general consensus the best radar detectors are:
Escort Redline/Beltronics STi Magnum
Escort 9500ix/Beltronics Pro 500
Overall the Beltronics STiR+ is the best, but it's also the most expensive at $1200 plus installation. Very good sensitivity, good "spread", good protection against QT (Quick Trigger), GPS enabled to let you lock out false alerts (very useful in an urban environment!), also able to disable certain frequency bands (only 3 Ka bands are used in the US) so detection time is improved since the RD has to sweep across fewer bands of frequencies. This is called Band Segmentation (BS).
Escort 9500ci is the corporate cousin of the STiR+ and has almost the exact same feature set (they are owned by the same company), but doesn't have Band Segmentation so it's not as adapt at picking up very short radar pulses.
Valentine One is also very good. It's very fast, meaning it will pick up Quick Trigger radar pulses more often than the Redline or the 9500ix. Its range is not as long as Redline (but still plenty in most cases), and about the same or a little better than the 9500ix. But the V1 is much less directional so it's better at pick up signals to the side (i.e. around a corner). The downside is the V1 can be very noisy in an urban environment due to its sensitivity and the fact it does not have GPS lock out. Some V1 users swear by the arrows.
The Escort Redline/Beltronics STi Magnum corporate cousins have very long range detection, which is great for highway driving. Probably the longest range detection, on par with the STiR+. But it is not as adept at catching QT pulses as the V1.
The Escort 9500ix/Beltronics Pro 500 are again corporate cousins. They have good range. But the big draw here is that they are GPS enabled so in a noisy urban environment, you can lock out false alerts. This is great because you are not overloaded with alerts that you start paying less attention to it. When you get an alert, you know it's likely a cop. Like the Redline, it's not as good at catching QT as the V1. It's "spread" is also narrower than the Redline and V1.
Note that Escort and Beltronics RDs have a ExpertMeter display mode, which shows multiple detected signals, kind of like V1's bogey counter. Another display mode is called SpecDisplay, which displays the actual frequency of the radar wave. This is useful if you know what you are looking for.
STiR+ is the best if you can afford it and don't mind a fixed install. None of the portable detectors are perfect. Each have pros and cons, depending on your situation and usage pattern. If you mainly drive in an urban environment, consider a GPS equipped model (9500ix, Redline, 9500ci, STiR+) to cut down on the false alerts. If you drive mostly highway, and want the longest range protection, consider the Redline or 9500ci/STiR+. If you drive more highway than city, and the cops in your local area use QT a lot, consider the V1 or STiR+.
Note that none of the detectors will save you from laser. Laser has a small dispersion radius, so basically by the time your detector has detected it, the LEO has already clocked your speed. Get a laser jammer (if legal in your area) if you want real laser protection. Laser Interceptors are highly recommended around the forums.
Also, nothing will save you from Instant On if it's targeted at your vehicle. This is when the LEO keeps the radar off until you are close, then target you specifically. There is no protection against that. Your hope is for the LEO to target the cars in front of you, and for your RD to pick up the reflected radar waves. So be very careful if you are the only car on the road!
A RD is a tool and does not guarantee a ticket free ride. Long time guys on the countermeasures forums will tell you, speed long enough, you will get a ticket regardless of what kind of countermeasures you have. Be smart. Stay alert. Be aware of your surrounds goes a long way. My last two speed tickets are from a LEO pacing me from behind at night (no radar) and a laser speed trap from a LEO on motorcyle (very hard to see on the side of the road).
Lastly, I personally run a hardwired Beltronics Pro 500 due to the GPS lock out ability (I drive almost only in the city) and better warning ramp up than the Escort equivalent.
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