CrustyEyeBalls Depends, here's a list of some of my favorites
Cross Platform, Free, Easy to use, and fairly reliable. But lacks some of the features I look for.
Pretty nice, you have to use the cracked version these days, it is comparable to Blackshades in reliability, ease of use, and speed.
An overall nice RAT, it has some issues such as remote desktop/audio/camera not working in some instances but it has a very nice GUI
Of course I have to throw my own in, It uses some of the most useful and reliable features from other RAT's and packs them into one instance. It is fast, reliable, easy to use, but still has some bugs as it is in beta with an extremely limited number of copies floating about.
If you are interested in R34P3R RAT you can ask for a beta copy on my forum! DarkComet is alright but be prepared to lose a ton of clients, it is super unreliable. nJrat is awesome, you can also check out Loki if you want something you don't have to port forward as it is PHP