What is a good blood pressure while on medication?

It depends. BP targets are generally set by a national group called the joint national committee, and they are getting ready to release jnc 8, an updated set of guidelines. Currently, they recommend a goal of under 140/90 if treated, except for those with diabetes or kidney disease, when the goal is under 130/80. Untreated BP is ideal at around 120/80, because heart risk goes up over 125/75.
Optimally ≤120/80. Consensus expert panels convene periodically to answer this question, but usually ?120/?80 is ideal. However, there is some flexibility given age and other conditions and risk factors. BP is quite variable naturally and subject to considerable measurement error, so calibrated arm (not wrist or finger) BP cuffs, good technique are important. 24 hour automatic BP monitoring is useful if in doubt.