Bone Hill has be far the most riding, is easily found and you can park right next to it
Other east Dartmoor venues worth a visit would be:
Hound Tor
Saddle Tor
If you end up further west down nearer Plymouth, then head up to Pork Hill and simply park up where ever and pick a tor!
These will all entail a longer walk-in, but are all worth a visit
Around Burrator Resovoir you've got Sheepstor, Combeshead Tor and Down Tor. All good for a laugh, but again a decent length of walk-in