Concerning sensitivities a lot depends on your mouse, mouse pad, screen (size?), resolution and personal taste.
Basic's would be to make sure any mouse acceleration is off (In CS:GO, Windows and at a driver level), make sure Raw-Input is on in game.
Windows sensitivity Stage 5 or 6.
DPI wise 400-1000 works (I seen a lot of pros using between 400-600).
In game sens between 1 and 2.5 depending on windows sen.
Polling rate I just stick on maximum your mouse can support.
I'm currently at 900 DPI, 1.6 Sens in game, windows stage 6, 1000 polling rate, and that is too high. I know its too high but I really cba adjusting
Lower sensitivity is generally accepted as better. I've heard talk of people saying if you do one swipe left to right of your mouse-pad and do a full 360 degree turn you are in the right area.
However at the end of the day as long as your sensitivity isn't too high you're gonna be OK.
I see a lot of people talking about resolution, a lot of pros are using 4:3 stretched, some using 16:10 aspect ration. There is really no right answer here and personally I'm not sure. I've always used my monitors native resolution in CS, since the early days and I cant say it makes a huge difference swapping about for me personally.
Also make sure Global shadow quality is set to the highest (Shadows are a big give away in some situations).
Effect detail on high lets you see through Molotov's smoke effect easier.
Shader I've also got on the highest.
Not noticed any difference in having Model/Texture detail on low/high so i just have it on highest.
Color Mode: Television (a lot use this, makes everything brightttter, far less dark spots).
Make sure motion blur is off (think it is by default).
Texture filtering on lowest.
Absolutely no AA/MSAA/etc of any kind (adds input lag).
Vsync off.
Finally;
voice_scale 1
This command is how loud you will hear your teammates through ingame voice chat, 1 being same as game sound, 0.5 being half of normal game sound. Helps against them raging russians when u try to clutch.