If you're after big time superhero stuff, there's two which totally changed the playing field yet are easy to get into, and both written by the same guy. Grant Morrison's All-Star Superman and JLA are simply amazing comics that just grab you by the balls and fling you into pure comic heaven. JLA brings you galactic threats and a non-stop high octane storyline, and All-Star Superman shows you prettymuch what the perfect comic could be. Morrison's other stuff gets pretty out there, though Vimanarama and We3 are straight forward enough and totally fun!
But All-Star Superman is just a MUST READ for anybody, even if they don't like comics, they should be made by law to read that.
My idea is to suggest anything by the king of comics, Jack Kirby (peace be upon him), however for many newcomers, his stuff can be really daunting, because it's about 500 different new ideas thrown at you on each single page, written in a wagnerian style with artwork so jaw-droppingly orgasmic, you'd have to cover your eyes with sunglasses just to look at the cover. When I rule the world, every island will have a mile high statue of Jack Kirby made out of solid gold, and he will be high-fiving a mile high solid gold statue of Frank Zappa (still can't get over the fact that my two idols were also good friends

). Kirby's kinda like, if you get him, you're in love. If you don't get it, you don't like and refuse to read (my brother refuses to read Kirby, even though he loves many of the concepts The King introduced)
Ultimate Spiderman is a good run to start if you're after a kinda generic superhero stuff. Nothing of any depth or content just some pretty boys and pretty girls flying around. It is pretty fun, but gets really shitty after they change the team behind it, and avoid Ultimatum like it's the fucking plague. One of the worst things ever written.
Justice League International (not the modern relaunch, but the old 90's one) is a good lighthearted well drawn and well written comic series which I would recommend reading as it introduces you to a load of characters, and is totally lighthearted and never takes itself too seriously, leading to such timeless scenes as the Batman vs green Lantern 'one punch' moment.
I know from other people, that Spawn had helped them get into comics, and to be honest, it's quite allright. It's grim, gritty and 90's enough for kids who think they're cool to get into, but has the amazing artwork of Todd McFarlane, which basically looks like somebody has vomited after just eating a bowl of Shreddies, and used that to draw the images. Spawn has served as a good introduction to many people.
You may notice that I am heavily promoting DC Comics, the reason for that is that Marvel haven't really done anything of much merit in many years. Their G.I. Joe run (1982-1995 or something) was their best non-Kirby comic, and likewise Marvel UK's Transformers were also well written. However Marvel have mostly been obsessed with rehashing their old ideas and haven't come up with anything new or exciting. Whenever anything interestign EVER happens in Marvel comics, it is always just A CLONE or A DREAM or something. Don't waste your time with Marvel, they became naff when they lost Kirby.