Guys which is better, bench press with bar or dumbells.
My chest is weak so i want the best exercise.

They're both good but if you really want mass, go with the bench press because it allows you to go heavy while placing relatively less strain on the stabilizer muscles than the DBs will. However, at some point you should also incorporate the DBs so your stabilizers can catch up with your strength increases and prevent injury

Another question, which's better exercise for chest incline vers decline

Go for incline you will reep more benifits, if you need to do anything for lower chest I recommend dips.

Both have their pro's and cons. I feel i have more control over DB benchpress though so i incorporate into my routine more than i used to.