Of course Rand would be the best choice to go against Hillary and here is why:
He has name recognition partially borrowed from his Dad's
He has the advantage of having a national campaign network set up by supporters of his dad which is pivotal to winning a primary.
Ultimately though everyone knows he is not his dad, so he has a much wider appeal to voters than just libertarians.
He is incredibly pragmatic focused. Even party purist can appreciate that.
He pointedly has made efforts to expand the party base among more diverse voting demographics.
And those are just the superficial advantages he has a contender. Here are his specific ones he has against just Hillary...
He is her kryptonite. When democrat and moderate independent voters are typically anti-war, Rand is the only GOP candidate that makes the most radical liberal in Washington look like a War Hawk.
Being one of the only Doctors officially in the race right now, he has a unique authoritative appeal in an election that inevitably health care reform will become a campaign issue.
Marco Rubio would be a good pick, even if Rand may be the better option. Rubio would have a better chance at Latino and Cuban-American voters than Rand Paul. He would also be able to contest Hilary on the "First" front because while she would be the first woman, he would be the first Cuban-American.