Now that the Republican presidential nomination is no longer in doubt, attention is starting to focus on the next big question to be answered in 2012: who will be Mitt Romney’s running mate? The main candidates for the job are well known: Marco Rubio, Paul Ryan, Rob Portman, Mitch Daniels and Bob McDonnell. But the only thing we know for sure is that unlike in 2000 when George W. Bush ultimately tapped the person running the job search —Dick Cheney — for the position himself, Romney won’t be asking his longtime advisor Beth Myers to put her own name at the top of the list she will be vetting.

With months to go before we find out the answer, anybody’s guess is good as any other as to the identity of the GOP veep. But Michael Barone points out that those expecting any great contrast between Romney and his choice or an attempt at balance are probably barking up the wrong tree. Romney would probably be best off picking someone like himself: a competent moderate conservative who would give the Republicans a “double vanilla” ticket. He’s probably right about that, but the only argument I have with this view is that one of the quintet of most likely candidates is anything but vanilla. If Romney were to choose Paul Ryan, he would be adding one of the most dynamic and ideas-oriented politicians in the country.

Analyzing this list, I agree with Barone that Marco Rubio should be taken at his word. If Rubio says that running with Romney “isn’t going to happen,” I assume that is going to be the case. Moreover, as Barone points out, the idea that Romney must have a Hispanic on the ticket to win is overblown. It certainly wouldn’t hurt, but it won’t make the difference between winning and losing.

Looking at the other four, both Daniels and McDonnell bring a lot of competence and intelligence to the ticket but not much in the way of charisma. Despite being as dull as dishwater, Daniels remains the idol of many conservative ideologues, but the same reasons that caused him to stay out of the presidential race are likely to keep him off of Romney’s ticket. McDonnell would help the Republicans take back Virginia, a key swing state, which will be a not insignificant factor in his favor.

In recent weeks, there’s been something of a boomlet for Portman. The senator from Ohio has the perfect background to appeal to a technocrat like Romney. As a former budget director and trade representative, he has the knowledge of key areas of economics that will be Romney’s priority if he gets to the White House. And though it’s not clear that he could ensure a Republican victory in Ohio, anything that would put that state back in the GOP column would weigh heavily on his behalf.

Paul Ryan also should appeal to Romney’s inner policy wonk. Ryan is his party’s leader on budget and tax issues and a powerful voice for reform of entitlements. Barone thinks his role, as chairman of the House Budget Committee, is so essential to the passage of any future legislation in a Romney administration that it constitutes the most powerful argument against him being picked to run for vice president.

But Ryan’s leadership on issues of substance also makes him something of a lightening rod for Republicans. Whether or not he winds up on the ticket, he will be an issue in the general election. Democrats will seek to demonize his reformist agenda and brand Romney as being as willing to destroy Medicaid as they claim Ryan is. To some that constitutes a powerful reason not to choose him, but if Romney is looking for a game changing choice for vice president, Ryan is the man.

For those who recall the last time a GOP candidate picked a “game changing” vice presidential nominee, rest assured that Ryan is no Sarah Palin. He’s among the smartest people in Washington and used to the give and take of debate in the big leagues of American politics.

Though all of the other potential veeps bring a lot to the table, Ryan is the top ideas person in his party and a perfect foil to Romney in the sense that he can’t be accused of flip-flopping on his principles. Far from hiding him in a congressional corner, Republicans would be well advised to put him center stage where he can wage the battle for conservative principles in the limelight. Ryan may be controversial, but he’s anything but vanilla, and that may be exactly what Romney needs.