The factors below should allow you to choose, as far as is possible, using the right sporting reasons, not out of sentimentality or by use of subjective criteria.
You should consider whether this person won the highest prize in their sport. More importantly, is the win of wider significance, or a one-off? Did it require excellence over a prolonged period? What was the requisite degree of skill required? Was there a relative degree of danger? Threat of serious injury is a psychological pressure only certain individuals can withstand; but this possibly includes some on this short-list.
Do not take into account what they look like, their human interest story, their background. In fact, any detail not related to performance should be disregarded.
You should not give weight to a person because the event in which they took part was exciting. This could be evidence of fortitude, but it could also be under-performance by a person who ought to have won more easily.
Rossi occupies in his sport the same position as Pele holds in football. For many MotoGP fans, including British, he is a demigod. I say this to put Rossi into context for those of you who do not know he is the most successful motorcycle racer ever.
Experts always make things look easy. Riding a motorcycle is not easy, certainly much more difficult than driving a car, running, throwing or cycling. Handling a motorcycle is very technical and the consequences can be severe if you make a small error.
MotoGP is the equivalent of Formula One without the in-race assistance, both technological and human.
To appreciate how fast MotoGP really is, you have to see it live. Though marginally less quick than F1, it has something that F1 often lacks – genuine head-to-head battles between riders, all of whom flirt with disaster.
When it comes to bravery, you cannot get much higher than riding at 200 mph with nothing between you and tarmac save for protective body pads. There is no steel safety cage or tensioned safety belt.
At these speeds, the slightest touch of wheels, or a mistake in braking can send a rider spinning through the air, praying his bike does not follow him.
In that environment, Rossi's winning of the MotoGP world title 2008 has put him in a unique position within motorcycle racing.
His 69th MotoGP win at Indianapolis broke Giacomo Agostini's record of most premier-class victories, set in 1976.
The title made Rossi the first rider to win the premier-class on four different types of motorcycle: 500cc, 990cc
five-cylinder four-stroke, 990cc four-cylinder four-stroke and 800cc.
He has also has won the world title at 125cc, 250cc and 500cc. This is the equivalent of winning four world titles in different boxing divisions.
It's not just a question of riding a bigger bike. It needs different handling characteristics, reaction times and other subtle, yet important changes technically and mentally. Many champions in lower classes fail to make the jump to higher ones. As James Toseland will attest, winning in one category does not guarantee success higher up.
Rossi, 29, was the only rider to score points all 18 races in 2008, demonstrating astonishing consistency, but it is not until you look at the Italian's career record that you realise that with his performances this year, he has dominated all in his path. When you read the statistics below, consider them not only against other riders, but also drivers in F1.
In a 12-year career, covering grands prix in four different classes, and 207 races, Rossi won 96; nearly one out of two (this includes the early years as a novice), he was in pole position 51 times, almost one in four. He was on the podium 148 times, nearly three out of four races.
See what I mean?
These amazing numbers must take into account that in 2004 Rossi switched teams. This is not just simply getting on a new bike. A different set-up was required to be absorbed, differences in every aspect of planning and preparation and working on an unfamiliar piece of equipment with mechanics that he did not know – all in the brief off-season.
Rossi's debut victory for Yamaha at the opening race of in South Africa made him the first rider in history to take back-to-back wins for different manufacturers.
That this had never been done before, despite the legendary riders that went before, including Barry Sheene, shows how difficult this challenge was.
You may think all this is sufficient to give Rossi your vote now but this year Rossi has also demonstrated he has potential on four wheels.
Rossi tested a Ferrari Formula One car at Mugello in Italy following his MotoGP title and completed nearly 30 laps with fastest lap time of 1min 22.5sec. Ferrari's last Mugello F1 test driver, Kimi Raikkonen, posted a top time of 1-21.079, only 1.5sec quicker. And Rossi's session was shortened by bad weather.
In contrast to the smooth tracks of F1 and MotoGP, Rossi finished 11th in the notoriously tough Rally New Zealand 2006 without regular rally driving.
Just occasionally a sportsman comes along and sets himself apart. They move the whole sport forward and everybody, whether friend or foe, agrees his iconic status.
Even if know nothing about, or dislike bike racing you must recognise the titanic sportsman that is Rossi.
If you vote dispassionately, free from preconception, considering what really counts for a person to win this accolade, you should vote for Rossi.
Many of the nominees fulfil many of the criteria that should decide the Sportsperson of 2008; none fulfil as many as Rossi.
Of course, you could always be British and vote for the nicest person.