Conical 5mm Wide-Angle LEDs are the most durable and brightest Christmas lights available.
Almost every professional Christmas light installer uses these lights for their displays. They are waterproof and snowproof because the LED molded into their plastic cordage (via injection). Conical LEDs are also 25% brighter than an incandescent indoor Christmas light. The brightness comes from the intensity of the LEDs, and this illumination is magnified by the conical lens of the lights.
Overall, LED lights are a great choice for Christmas lights because LEDs almost never fail, and they are impervious to freezing temperatures. They are also much more energy efficient than incandescent bulbs. For this reason you can string up to 40 sets of LEDs together, and plug them into a normal electrical socket.
Conical 5mm LEDs are available from Amazon and eBay.