Shimano's 105 Braze-On Front Derailleur sports extra-wide beefy pivots, which eliminate flex for lightning-quick shifts. Plus, the clear-coated aluminum body and chrome-plated-steel cage will last for years.
Also available in Slate Black (image unavailable from manufacturer).
Braze-on front derailleurs mount to either a built-in frame mount (left) or to a clamp mounted around the seat tube like the one on the right.
Capacity is calculated by subtracting your smallest chainring's tooth count from the largest chainring's tooth count. Example: 53/39 chainrings: 53 - 39 = 14-tooth capacity.