This article is from the Radio Control (R/C) Flying FAQ, by Shamim Mohamed firstname.lastname@example.org with numerous contributions by others.
That depends on what you want to do and where you're starting from.
If you've never flown RC before, and you want to start with an electric
plane rather than a pure glider, I recommend an electric glider like the
Airtronics Eclipse. This will give you the best chance to stay ahead of
the plane. In the sport/trainer category, I hear a lot of good things
about the Leisure Amptique.
If you know how to fly RC, you have a lot of choices. The simplest and
most available electric power systems use six or seven cells. These are
called "05" systems, and are very similar to the power system of an RC car.
You can find all sorts of planes in kit or plan form which will work well
with these systems. Outstanding examples are the aforementioned Eclipse
and Amptique, old timers such as the Leisure Playboy and Astro Viking, a
variety of semi-scale kits from Davey Systems, all sorts of gliders, and
the aerobatic ElectroStreak from Great Planes. Any two-meter glider kit
can be easily adapted to six or seven cell electric by a moderately
competent builder. Just stick a motor in the nose, battery under the wing,
If you want more performance, good ground handling, or just like larger
planes, there are larger power systems available, all the way up to systems
which will handle a 60-sized power plane. The cost and complexity, of
course, go up with size. Any reasonably well-designed power plane kit or
plan can be adapted to an appropriately chosen electric power system. The
first step is to leave out half the wood---all power planes are grossly
overdesigned. Electric motors generate very little vibration, which helps
you get away with lighter structures.