When looking at the best options for weapons, there are a few to take into account. The AWP is the strongest and most expensive single shot sniper rifle. Every shot which you hit above the legs is an instant kill, unless of course there is a wall in the way. A leg shot will usually deal about 85 damage to an armoured target. You can buy this weapon regardless of which side you are on.
On the Terrorist side, the AK-47 is the usual weapon of choice. With a one shot, one kill when hitting someone on the head while still being affordable, the weapon is one of the most used in the game. The spray itself is quite controllable and definitely one to master if you want to improve in CS:GO.
At the Counter-Terrorist side you have two options though. The M4A4 or the M4A1-S, with the S version being slightly cheaper. However, with this slightly cheaper option come less bullets but an easier to control spray. The M4A4 packs a bigger punch but will be harder to control for most players, as it also has a slightly higher fire rate.