What is the best medium for sending feature requests? Both for Ubuntu as a whole and individual (supported) apps?
For individual apps, the best way is not Brainstorm, but filing a bug requests or feature request in the individual applications.
Some upstream development portals might be having explicit feature for asking user ideas but most of them use their bug tracker for that.
Example, you want a new feature in
Or before filing bugs for wishlist, you might want to talk to the devs of the particular application. You can look at their mailing list or IRC channel.
For Ubuntu, as Octavian mentioned Brainstorm is supposed to be a good platform, but sadly most of the ideas are neglected.
Recently Canonical CTO Matt Zimmerman mentioned that
The Ubuntu Brainstorm Site was set-up for just this purpose. Users could suggest new features and others could comment and vote on these. This site is not in use any more and in practice when it was few suggestions made it into Ubuntu.
Depending on the type of request reporting a bug on Launchpad may be appropriate. If its a request for a minor change file a bug stating its a "wish list" and what you would like. Wish list bugs tend to be low priority though. It may be better to try and find the upstream developers for that package and contact them directly.
If the feature: is can we include this package in the repositories you file a bug for that too and tag the bug "needs packaging" see here