First let me state that the facts and opinions I'm going to give are from personal experience. I've used all 3.
Tango Down - It's thicker than the KAC but a wee bit thinner than the CQD. It has internal water proof storage and a pocket for a tape switch. It can be attached and removed without tool but it has to slide onto the rail from the front. Designed for mil-spec rails, it will have to "tweaked" to fit on non-spec rails. This also means it might be a little loose (I do mean a little) on some rails. Since it's plastic, it won't get hot from weather conditions or extended firing. It weighs 3.6 oz.
CQD - Without a doubt, it is the most robust VFG available. The rail it is attached to will probably be destroyed before it is. It's constructed of aluminum which means you may want to either use gloves or wrap it in paracord (I did the later) when using it. It attaches to the rail with a cross clamp and 3 screws so it attaches from the side vs. sliding on from the front. The base is pretty wide. It takes about more "rail estate" than the other 2 mentioned here. It has a waterproof internal storage compartment and slots for wires that lead to the slot for a tape switch.
KAC - This is pretty much the one that all others are judged by. It's has the smallest diameter of the three which some prefer, especially when using gloves. It slides onto the rail from the front and is secured on the rail by tightening the bottom which pushes the nub at the top of the core in the rail. This is a very secure connection which does not require tools to remove. Like the TD, it is constructed of plastic so it won't get hot. While it has no internal storage or tape switch slot, these things can easily be remedied. If you need storage, you can get a replacement core from GG & G. If you want to put a tape switch on it, you can use a piece of bicycle inner to hold the switch in place.