+1

Depends on what you wanna do:

If you want to race? How far up the ladder do you wanna move in racing, amateur? Non-licensed? Licensed? Category 5, 4, 3, 2, or 1? Professional? Local pro? Regional pro? International pro? European pro?

Which races, what kinds? Flat time trials? Long time trials? Hill climb time trials? Criteriums? Downtown or industrial park? Road Races? Point-to-point, loop, or circuit races? Stage races? Cyclocross? Mountain bike?

Or do you wanna just ride? By yourself or with a group? Local club rides? Long distance club rides? Fast group training rides? Organized events? Centuries? Half-centuries? Double centuries? Paris-Brest-Paris?

Do you ride track? What events? Pursuit? Miss-n-out? (Add here a half-dozen others I know nothing about.)

Then, what terrain do you wanna ride on? Flat Florida coastline? Rolling hills of the central US? Short, steep hills of West Virginia? Extended, 8-mile climbs in Colorado or California?

Even then, there is no ideal body weight. Miguel Indurain, for instance, was tall and about 175-lbs. when he was winning 5 Tours de France in a row. That's extremely heavy for that type of a race. But he did it in the flat Time Trials, usually (but not always!) just hanging on in the mountains. Lance Armstrong only weighed in at 160--165 lbs. and Greg Lemond--shorter than Armstrong--probably a little less than that. Andy Hampsten, also a short rider, was an excellent climber and won the famed Alpe d'Huez mountain stage at a paltry 135--140 lbs. Mark Cavendish at 150-lbs., is small for a sprinter, but currently wins most sprint races with ease. Tom Boonen, also a sprinter and classics rider, is 180-lbs.

With all these variations, I hope the answer you can see coming: there is no ideal body weight!

What it comes down to is: A good body weight is one that is healthy and lets you achieve your goals.