The best RTS mice--at least if you play competitively--will have perfect tracking at medium sensitivity and be relatively light weight to ensure high apm. Stability of the mouse while clicking and double clicking is also very important as you will be trying to get at least 2 APM a second and want it all to be effective apm and not just spam. Also, things like fancy firmware/software are avoided like the plague. Reliability is way more important than bells and whistles.
I have owned/used a MX518, G400, G9x, G500, G700, Abyssus, Death Adder, Imperator, and RAT 7. I have also used a Razer Exactmat, Steelseries QXC+, 4hd, Razer Goliathus Control, and normal cheapo mouse mats with each of these mice.
Of all the mice I have used, the G9x and Abyssus by far are the best for RTS. I can see why some might prefer a G400/G500/Death Adder as they are relatively light mice that are good for palm grippers but I am more a finger touch gripper so they are not for me. Also they are relatively heavy.
The G9x and Abyssus are both small mice that can support claw or fingertouch grip. They have excellent tracking (G9x has slight acceleration as it uses the annoying Avago 5700 dpi sensor) and they have good switches. I have a Steelseries Sensei and Zowie AM in the mail so I will report on those as soon as I get them. As always, it really depends on your preferences. I prefer an abyssus on a QCK+ at 1800 hardware DPI and 6/11 in windows. It is a medium sensitivity on a slight friction mouse pad. Some might use, for example, 450 DPI on the abyssus on a 4HD (Slippery plastic mouse pad). It all depends on preference. I was happy with my g700 and g400 before until I got into high diamond in starcraft and my control was just not precise enough with my old setup. Misclicking even one focus fire would ruin the game.
Given that, I think any heavy mouse such as the RAT series and the G700 (which I love to death in every other type of gaming genre and general productivity task) is a HUGE mistake. Look at what the top pros use. In SC1, they use mainly the G1, a very light old optical mouse. In SC2; SKMC uses a kinzu, IM_MVP a G9x, and all the Western pros use a mix of death adder, abyssus, and Steelseries Sensei. Granted most of these pros have sponsorship deals but razer and steelseries produce a gamut of mice and the pros all choose the lighter mice in general.