It seems that the mass of the Space Shuttle at launch was around 2,000 tons, and it could deliver a total payload of around 122 tons to LEO. This would give it a launch/LEO mass ratio of around 16.4. Is this the lowest ratio among all orbital launchers in history?
The shuttle was certainly one of the more mass-efficient orbital launchers, but it's a more complicated question than it might seem at first.
The shuttle orbiter is ambiguously part of the launcher and part of the payload, so it's simply not possible to compare it apples-to-apples against other launchers.
For "pure" payload that's going to stay in orbit after the flight, the ratio is very poor, as it delivers about 25-27.5 tons. This considers the orbiter and crew to be dead weight.
If the crew are going to be doing useful science then it's reasonable to consider the 68-ton orbiter as payload, bringing the delivered payload to 93-95.5 tons.
The ET isn't useful and doesn't reach stable orbit, so it's not reasonable to count it as delivered payload. Building space stations or fuel depots out of ETs has been proposed, but these plans were never developed.
This gives us a practical payload ratio of about 21.2:1. This is just about on par with Saturn V's demonstrated 21.7:1 (Apollo 11; later flights may have performed slightly better).
Falcon 9 Full Thrust has a ratio of about 26; Atlas V 401 has a ratio of about 32.
I will also take this opportunity to note that defining efficiency by the ratio of liftoff mass to LEO mass is only of academic interest; liftoff cost to LEO mass is a much more significant figure of merit.