Are you looking for resale? Are you reading the books? Are you just looking to speculate?
You need to be a little more clear on what your goals are. It would help us advise you better.
Whether trying to build a nice collection or getting books before they get too high for you to afford, I would learn/look up what the key Hulk books are. That way you can buy first appearances you like, the best looking covers, etc. I honestly don't know much about Hulk books except #181, which is the first appearance of Wolverine.
Actually, IH #180 is the first appearance. Ya, I said it!