Q:

The source free RLC series/parallel circuit

If the energy is dissipated more quickly in overdamped circuit then why it does have longest settling time?  

A:

This answer uses the example of a series capacitor, inductor and resistor and assumes the capacitor is previously charged to some voltage level. 
If the over-damping is massive i.e. a massive series resistor then it dominates the inductance and the circuit becomes a resistor discharging a capacitor hence, the discharge time can be very long if R is very large. 
As R reduces, the discharge time reduces.  But as you approach critical damping (Q = 0.5) from the over-damped case, the decay changes from being a simple exponential situation to a decaying ripple where the frequency of the ripple is a decaying sinewave having a frequency of \$\omega_n\sqrt{1-\zeta^2}\$, where \$\zeta = \frac{1}{2Q}\$ and \$\omega_n\$ is the natural resonant frequency. 
At Q = 0.6 (slightly less than critical damping), the decaying sinewave is very limited in its extent.  The overshoot of the first cycle is about 0.9% and, to most folk would hardly count.  As the sinewave decays, the first undershoot is about 0.008% so it's pretty much over as soon as it began. 
The ratio of the magnitude of the 1st positive peak to the first negative peak is approximately 0.9/0.008 = ~112. 
However, as Q rises more and more, the overshoot and undershoot become more dominant.  For instance at Q = 1, the first overshoot peak is about 16% and the first undershoot peak is about 2.7%.  You can see that these are closer in magnitude than was the case when Q was only 0.6.  Ratio is about 6. 
As Q rises even more you eventually reach the situation where the overshoot and undershoot continue for a very long time.  This means that energy is kept in the reactive components for a very long time. 
Conclusion: The energy is dissipated more quickly in a series RLC circuit at around critical damping.  Either side of critical damping it takes longer and, at extremes (R = infinity, R = 0), it takes an infinite length of time.