Parts of a Novel

Which part of your novel is the most important?  The beginning?  The end?  The plot?  The characters?  Well if you are a reader you likely already know the answer is "all of the above".  As a fiction reader you want to be entertained by believable characters and a thrilling plot.  Popular books require good characters and a beginning that grabs our attention.  The ending is not as critical, since it could be delayed over a series of books.  But eventually the writer is going to have to consider an ending that satisfies the reader.  The longer we wait for an ending, the better it needs to be.
As a side note:  just because the world is moving to online books and online reading, where readers can discover free books online does not mean we can ignore the fundamental need for good writing.  If you want to read a novel, whether it is online fiction or in a book; you want your time to be well spent.
Romance fiction may not need to rely on a plot as much as a science fiction story, but the reverse could be said for good character development.  Each genre has it's strengths and weaknesses, but if we focus on writing a well-rounded story with good characters then we can be sure we will entertain the reader.
The first sentence.  Unless you know you have an amazing first line, be assured yours will change by the time the book is complete.  Research "fiction first line" on the web and see what I mean.  There are some great first lines from amazing novels.  A first line grabs attention and makes the reader want to keep reading.  Or maybe it is intentionally misleading, or prophetic.  Perhaps all you have is a great first line - well that may be enough to get you to create great literature.  Write that great first sentence and then see what happens.
Develop the characters.  Real people are difficult to describe in words.  The author needs to visualize them and know them, know how they will respond to outside influences and emotions.  How do we describe our character in the novel?  I gave this assignment to some students once and got very two-dimensional descriptions like tall, fat, ugly, nice, mean.  It does not good to describe your character as tall.  How tall is she?  Is she taller than every boy she ever danced with?  Was she self-conscious about her height, and this made her shrug a little?  How do we know the boy is nice?  Do we have evidence of it shown in some action?  Making the reader infer qualities rather than telling them makes the difference between a children's book and a real novel.
Literature is an art and creating it makes the author an artist.  There is certainly no way to say this is for sure the right way to do things and that is the wrong way.  Focus on creating a great first sentence.  Have a plot that makes sense an can be followed logically.  Create characters that are believable and that our reader can identify with - so that they cheer for the character (or cheer against him is good too).  You are on your way to writing a great novel.