When it comes to weight training, the best weight is going to be the heaviest weight you can do all the reps in a set while maintaining proper form.
It is important that the last few reps challenge you, but you should still feel like you can complete them safely and correctly.
You should increase the weight if you feel like you can easily keep going at the end of your set. So the last 7-8 reps of the set should be quite a challenge.
Consider reducing the weight for that exercise if halfway through you feel the weight is too heavy or your form is compromised. It is CRUCIAL to use the correct technique when working out with weights to prevent injury.
If you use a weight that is too heavy, you put yourself at risk of injury.