The space vehicle with the largest interior pressurised cabin volume is the International Space Station with a pressurised volume of 931.57 m3. It's habitable volume of 388 m3 is only slightly larger than Skylab, which had a habitable volume of 351.6 m3.
Habitable space tends to be limited to the minimum in orbiters and proposed interplanetary craft to reduce the mass, requiring less fuel or being able to use current rockets to launch the space craft. Space stations are able to be larger as they are built in modules, their subsequent sections are combined to calculate the total final volume.
List of manned spacecraft and their pressurised/habitable volumes past and present. All volumes with a * are habitable volumes (usually significantly smaller than total pressurised volume).
- ISS: 931.57 m3, 388 m3*
- Tianggong-2: 14 m3
- Soyuz MS: 10.5 m3 (Largest Soyuz craft according to this table)
- ShenZhou: 8 m3
Bigelow Aerospace has proposed a possible space station with a habitable volume of 3000 m3. This would be by far the largest manned spacecraft.
† The Shuttle habitable volume only includes the basic crew cabin, without any additional habitable modules (as pointed out by @OrganicMarble in the comments)