Never saw Forrest Gump. Zero interest. Always looked like bloated over the top Hollywood pablum to me...
My default answer is usually Goodfellas, as I like the genre, the acting, and I like movies that have some basis in reality. I'd probably put Rocky and Jaws up there as great classic movies. Raising Arizona is just a perfect movie from the way it was shot, to dialogue, to plot, to acting. Shawshank is great too. As far as movies I just flat out enjoy and watched over and over again as a kid in the '80s...
Airplane!, Vacation (only the original), Sixteen Candles, Weird Science, Easy Money, The Toy, Fast Times at Ridgemont High, Porkys 1&2, Star Wars, Empire Strikes Back, Return of the Jedi and many more that I'm blanking on.