Lenses are always one of the first things I personally upgrade. I am a big fan of the Chromatic lenses as they are the flashiest, and also provide great shading from the sun, which is important when playing outdoors. Smoke lenses are also a great way to block out the sun, and will not show a glare, meaning you can be sneakier. If you play indoors most of the time, it is a great idea to pick up one of the High Contrast lenses, as they can help you differentiate between the landscape, and your enemies. And of course you should always have a clear lens just in case!