In your opinoin, what is the best exercise for the back? - to shapen and strengthen the back, and to improve one's posture?
It really depends on your back development. If you lack width, you should do wide-grip chins/pulldowns. If you lack thickness, you should do some sort of row.
Recently, I've only been doing close grip chin-ups. But now, I'm considering doing Bent-over rows instead... or should I do both?
Do both. Variety is the spice of life.
Close grip chin-ups and bent -over rows both involve shoulder extension
, so for the upper back, there is some equivalence (although the chin has a longer range of motion). Bent-over rows, however, involve the lower back while chins don't.
Wide grip chin-ups involve shoulder adduction
, which in most people is a weaker movement than shoulder extension. The posterior deltoid is a synergist in shoulder extension but not in shoulder adduction, so people who work the back primarily with wide-grip chins might need more posterior deltoid isolation work.
Wide-grip chins/pulldowns, bent-over rows and deadlifts are an excellent back routine. The only problem is that the lower back might get overworked if you aren't careful.