>>116885402God, I used to love Marvel. Hulk used to be the epitome of it's weird sci-fi goodness. To know the OG Hulk is to know about OG Marvel. For a real comprehension of the character, here's what I'll recommend you as a life-long Hulkster.
Start with The Incredible Hulk: Marvel Masterworks. It contains the very first Hulk stories and will show you the character's initial persona, introduce the most important key cast of Hulk's life, and much of the oddities that occur in them (such as the Hulk's initial grey coloring) become very significant later on in future stories written even years later.
Then, I recommend getting Marvel Essentials Incredible Hulk volumes 2 and 3, which is right about where the the Hulk starts to become the jolly green giant we know today. Here, Hulk's rogues-gallery is further defined. If you want more of the fun and strange savage hulk, move on to volume 4, because it has an important events regarding Betty Ross, Glenn Talbott, and the introduction of the Wendigo. But the quality is a little bit all over the place in volume 4. From there read more silver/bronze age stuff...if ya want, I guess.
Next real big saga is Peter David's run. It went on for over a decade and is one of the pillars of how the hulk works in general. His run begins with the Hulk grey again. Starts in 1987 I think. His whole run should be in the Peter David Hulk Omnibuses 1 & 2. They're expensive; try reading it all online.
From that point on, I think you can read The Bruce Jones/JRJR run. It's really weird, but not really bad. I just remember it like a fever dream.
Then you can get into Planet Hulk, World War Hulk, and the gay Red Hulk Saga. After that, you're caught up before Marvel turns gay as shit. I don't know much about Immortal Hulk because I stopped reading Marvel in 2015 when they went full unbased, but it DOES draw from Peter David's run.
God, I can't believe I turned my back on something I used to adore so much. Fuck comics. 
