For Red and Green, both originals and remakes.
For Gold and Silver, I'd stick with the remakes. The original just isn't good at all to be honest with you guys. The biggest flaw has to be Kanto. Kanto got nerfed so that you are unable to explore Seafoam Island, and Cerulean Cave, and Viridian forest has been nerfed to being a part of Route 2 other than its own area, which turns me off disappointingly. The remake gave it a huge improvement (especially Kanto being what it was meant to be unlike the original) with the addition to the new feature such as Pokeathlon, and the Safari (The original being the only games prior to Gen V to not have Safari). One thing that sucks about the Remake is that the phone feature from the PokeGear, especially when it comes to Trainer Rematches is still kept which, *frustrated sigh* you're going to train at Elite Four a lot to level grind (which is faster to get to level 100 unlike the original, since after the first time, you get to fight those that are level 60+, and good if you farmed Lucky Eggs at Route 13) and money farm, and you get annoying calls from trainers such as Youngster Joey who bribes about his "Rattata." So yeah, I'd stick with Heart Gold and Soul Silver.