As a first army marines are great. Their stats and units are easy to memorize and can help you learn the rules of the game. Also, if you start getting bored with them, you can start adjusting them to become a dark angel, blood angel, black templar, space wolf, or sometimes even a chaos army. The chaos models and units are cool too, but as I said before, if you didn't get any Land raider crusaders, scouts, landspeeders, or whirlwinds in a normal marine army, you could easily make the army chaos by changing their banners, commander, and maybe adding some spikes here and there. If you want to learn how to play and use a very easy to handle army, marines would be your best bet. Chaos is a close second, but although they have a ton of different, cool looking, units, some of those units are considered useless or not worth the points by most people (ex. possesed marines). Meanwhile, the marines have whirlwinds, scouts, landspeeders, land raider crusaders, the They Shall Know No Fear special rule, and other special rules letting you combine your forces with a witch hunter or daemonhunter army. In short, a very adaptable army with tons of useful troops and vehicles. Chaos, as I said before, is a close second, but a marine army's ability to help you learn the game and then be transformed into a dark angels, blood angels, space wolves, black templars, chaos, witchhunter, or daemonhunter army is what I think gives them a slight edge over chaos. Imperial Guard I don't know much about, but I do know that their troops are weak and their codex is pretty old.