Which orbital class rocket (active or retired) has the highest thrust to weight ratio at liftoff?
I took the data from the JSR Launch Vehicle Database, available here. This includes take-off thrust and launch mass, so the Thrust-to-Weight Ratio was easily calculated. Unfortunately this list also includes many missiles and sounding rockets and I was too lazy too properly filter them with some other list of orbital class launch vehicles. However, eyeballing the list for orbital class launch vehicles with successful non-test missions, the Mu-3H seems to be the highest. It has a TWR of about 4.9, which certainly is quite high. Most of the Mu-family rockets have very high TWR.
After some further digging on the Mu-family I found a source in Japanese with sea-level thrusts and another (in English) with take-off mass for each configuration of the Mu-family rockets, which gives the following TWRs:
- 4.74 - M-3C
- 4.57 - M-3S/M-3H
- 4.52 - M-4S
Assuming that the question refers to thrust to "total vehicle" weight ratio, I would opine that the ratio is determined by the mission. A heavy launch-to-orbit vehicle would be loaded to get the heaviest load to space and have a low thrust to weight ratio. An anti-missile missle would have a high thrust to weight ratio to accelerate rapidly and get to an approaching missile as far away from it's target as possible. I don't see any value in this parameter as a figure of merit.