Darwin's Theory of Evolution by Natural Selection remains as the single scientific theory that more completely explains more functions of Life, in the most succinct format of any major theory of science to date.
Since I'll probably get flamed anyway now, let me share the single greatest insight, of many, that I take from this short, readable, and entertaining book:
Although we think of genetic replication as being full of errors (mutations), the most important feature of genetic DNA is it's "digital-like" accuracy at repeating extremely complex protein design over hundreds, if not thousands of generations.
In other words, it's not like making a copy of a copy of a copy of a video tape, that loses fidelity with every copy. A good design for an eye will be passed down flawlessly to millions of progeny over thousands of years. It's like copying from a floppy-disk to another floppy. We fully expect a 100% accurate copy of 1.44 million bytes from one to the other, and could use the copy to make another copy and another thousands of times with 99.999% perfect success.
Although there are errors, usually fatal, life would not have clawed it's way up from the muck if each "better idea" slowly eroded through poor copying of the complex DNA code that both makes life possible. That's why the primary function and payoff of life is to pass this extremely precious genetic cargo on to the next generation.
Viva life, and Viva the impassioned debate that drives us forward.