Browning/FN Hi-Power 9mm.
- Widely used by many military and law enforcement agencies for years. This implies parts availability such as magazines and springs.
- 9mm because of being a common round.
- Designed for service, very accurate, and reliable.
There is a close second place choice as well, but the OP says one recommendation.
If you find your back is up against a wall, maybe you have been backing up for too long.