No, I don't mean the American Express card. What cycling gear do you use constantly? What has been your most used, and hence best, investment?
Here are mine, from the ground up:
I don't ride clipless, for reasons I won't go into now, and I envy the additional power the stiff sole of a cycling shoe transfers to the pedal. So I use the Sette Rival II shoe. I wore them on both my trips from Pittsburgh to DC, and found them great for pedaling and for walking around. They accept SPD cleats if you want to use them with clipless. The only drawback they have is that they are a little difficult to get on and off - the stiff sole prevents them from just pulling on easily. But for 35 bucks I'll suffer.
Wool socks are a great investment. They are wonderful in any weather, and doubly so in the cold and wet. I got mine for about ten bucks at Performance, but many companies make them.
I always use gloves when riding. In cold weather, I love these Descente lobster gloves. They have a rubberized cover that pulls out to cover the outer three fingers on each hand. IIRC, they were about 40 bucks, and they've held up well through a year's use.
Whether its riding through Big Savage Tunnel, commuting at night, getting back at dusk from a grocery run, or just wanting to be more visible on an overcast day, a helmet light is a great addition to your skid lid. Mine is a Planet Bike model that has differing settings, including a blinker. It was about 25 bucks.
So, what gear are you attached to?