What would be the most reliable test for an acutely injured knee of a 27-year-old athlete -
The best test for an ACL injury is Lachman's test

"When the preoperative examination (no anaesthesia) was analysed for patients with isolated ACL injuries, the authors found that 98% of the Lachmann tests, 54% of anterior drawer tests, and 27% of the pivot shift tests were positive. When the examination under anaesthesia was analysed for these isolated injuries, authors found that 100% Lachmann tests, 81% of anterior drawer tests, and 100% of the pivot shift tests were positive". -- Evidence base sports medicine
Anterior drawer test
Posterior drawer test
Lachman test
Steinmann test