"Always second best." => 언제나 2등.
"Always second choice" => 언제나 차선 [둘째] (strict translation)
For "second choice", there are more widely used expressions:
- 언제나 찬밥 신세: Always the last choice (the unwanted one). 찬밥 = cold meal.
- 거들떠 보지도 않는 존재: Someone no one gives a damn about.