I will cover two things here:
1. Why is hulk unbeatable?
2. If hulk is unbeatable then why has he been beaten before?
Part 1: Why is hulk unbeatable?
(READ THE SCANS FOR PART 1 AND THEN READ THIS PARAGRAPH FOR PART 2) If I had to list all the scans and feats that prove that hulk is unbeatable, I would be here for a year... So I am just going to go to part 2.
Part 2: If hulk is unbeatable than why has he been beaten before?
The reason to why hulk has been beaten before is because writers don't want to make hulk a boring character. If hulk were to win every single fight he gets into, then he would no longer be an interesting character. People would stop buying hulk comics. A character is no longer fun if he wins all of his fights. For that reason, writers make the hulk lose some of his fights even though hulk should win any fight he gets into.