If you watch professional CS2, you will notice a massive percentage of players play on a 4:3 stretched resolution (like 1280x960) rather than standard 16:9 Native (1920x1080). There is no objective "best" choice, but you must understand the mechanical trade-offs of each.
16:9 Native (The Tactical Advantage) Playing on your monitor's native aspect ratio gives you the maximum Field of View (FOV). You will physically see more of the map on the left and right edges of your screen. This means you are less likely to get flanked by an enemy who is just outside your peripheral vision. The game also looks significantly sharper.
4:3 Stretched (The Aiming Advantage) When you take a 4:3 image and stretch it to fit a widescreen monitor, everything distorts horizontally.
The Pro: Player models appear significantly wider, making them visually easier to track and click.
The Con: Your FOV is reduced (you can be killed by enemies who are technically on your screen in 16:9, but cut off in 4:3). Additionally, because the horizontal axis is stretched, enemies moving side-to-side will appear to run faster.
The Verdict: If your aim is inconsistent, try 4:3 stretched to widen your targets. If you play a heavily tactical, anchor role where information and peripheral vision are key, stick to 16:9.