Golly gee, this is gonna be hard. I'm not going to assign numbers because I will think too hard about it, so just an overall top ten. I'll probably get pretty lengthy with the explanations.

The entire Ender-verse - Orson Scott Card - Most people I know have only read the first book, Ender's Game. I believe this is because its much more popular as a 'young adult' novel despite still being pretty heavy. The final 3 books in the Ender series are definitely different beasts, and really made me think about a lot of stuff. The Shadow's series is also really good, Shadows in Flight was a bit of a downturn IMO, but I think it will pick up with the next book that unites the two plot lines. 

The Homecoming Saga - Another by Card, This was a great series, not to cause any spoilers, but once I figured out it was based around the Book of Mormon(my first guess was Bible, I was close!) it became even more fun to read. Im not Mormon, but I do like the... I guess you could call it gimmick, that it provided. Last book in the series was fantastic.

A Grey Moon Over China - Thomas A. Day - There actually isn't a Wikipedia article for this one for those that haven't heard of it but might be interested, so I'll summarize with as few spoilers as possible. This is a somewhat technical hard sci-fi novel with an epic scope. It depicts a bleak war-torn(not apocalyptic) near-future Earth that comes upon a limitless energy source. The results of this revelation make a huge impact on the world, but I can't go much further without spoiling plot elements. If you are interested somewhat in geopolitics, hard sci-fi, and realistic human struggle, check it out.

The Mars Trilogy - Kim Stanley Robinson - I'll be brief, great series by KSR. Its about the exploration and colonization of Mars. Very hard sci-fi, but if you enjoy that kind of stuff like I do, go for it. 

Ringworld - Larry Niven - Niven is a titan of sci-fi, at least in my eyes. This is an amazing book, and its sequels are great as well.

-Of Worlds - Larry Niven - Another series by Niven, same universe as Ringworld, different time and place.

The Rommel Papers - Non-fiction, an account by Rommel himself(edited together posthumously, with a few people who were with him filling in gaps) of his time during WWII. I actually don't remember if it includes his WWI service as well, but I've read another book on that and it kinda blurs together.

Wheel of Time - Robert Jordan - Haven't actually finished it yet, but great so far.

The Passage(and its sequels) - Justin Cronin - Third book is due this year, holy shit this is amazing. Wikipedia mentions it being a 'vampire trilogy' which is a huge disservice to the books, as they are nowhere close to the spectrum of modern vampire novels, and the 'vampire' connection is tenuous at best. I recommend giving it a try, great book/series. 

For a tenth, ASoIaF by GRRM I guess. I was originally not going to include it despite how much I like it, but I was having trouble narrowing down a tenth.