Bryn Celli Ddu is about the furthest from the car that I would include, although Mein Hirion is always tempting ...
Penrhos Feilw takes you a bit off the circuit, but you're right in saying it's close to parking. Places like Din Dryfol are out of the question really, as it is both quite a walk from parking and a bitch to find where to park.
There are a lot of good sites on Anglesey that are 'close' to parking really, so a one day megalithoblasticofantastico(TM) trip can be very rewarding.