If just the one stem is that tall i'd reduce it to the same height as the others or all you'll get is the plant putting all its energy into that one stem and the others will constantly lag behind and be several feet shorter.

To make shrubs look attractive you need to keep all stems at the same height.

Forsythia's as tough as old boots and you can hack at them and they'll still grow a few feet every year, do the plant a favour and cut that tall stem back, you don't need to worry about where to cut it - anywhere will be fine.

Did you prune it after it flowered? I know that it's really the wrong time to prune it now, and any flowers on the long shoot will be lost, but to keep the shrub in balance, I think I'd cut it off - as you say, to new buds.

SNAP Louise! :-))

Don't like to argue with Spritz and Louise as they are both right, but in spite of that if it was mine I would wait and let it flower in the Spring and then cut it to the size you want as soon as it has finished. If you have a chance of lovely yellow flowers in March I wouldn't waste it! When you prune forsythia remember it flowers on the stems it produced in the previous summer so if you prune in autumn you will lose a lot of potential blossom. Also if you cut it now there is a danger it will grow more shoots that won't have time to ripen before the cold weather. But it sure is tough so not to worry.