Everyone has their favorite and so do I, I like my Light & Motion Vis 180 (not the Micro Vis 180), it cost about $100. That light has survived many downpours and even survived a fly off the bike onto concrete without a scratch, and it's so bright it looks like a road flare in daylight! But having said that even my cheap Cateye LD600 has survived the same stuff but it isn't near as bright as the L & M. Technology is always moving forward and today Serfas has a brighter light than the L & M called the Serfas TL80 that cost about $75, not sure about the waterproofness of said light but I doubt Serfas would put out a light that would fail in real world situations like rain!

Find a light you like for a price you can afford and you'll be fine, but try to buy something as bright as you can afford.