Ever since reading Sam Sheridan's 'A Fighter's Heart' I've been following the career of Andre Ward.
His recent fight against Carl Froch was a beautiful display of boxing. Also, he must be well informed about MMA because he trains with Nick Diaz. In my opinion his style of boxing would be perfect for MMA:
- Good range for his weight
- Angles, distance and footwork to avoid takedowns/kicks
- Strong inside fighting for clinch work
His whole game is built around picking opponents apart, even in the book there's a chapter on how his coach dissected muay thai to see how a boxer could use footwork to diffuse the strengths of kicking.
So - does Andre Ward have the best style of boxing for transitioning to MMA? Would you like to see him in the cage?
3 months sprawl training and he would be champ 
He would kill JDS and Cain in less than 3 minutes
Not even 3 months, maybe like 1.5 months of hardcore proper sprawl training, and he'd be champion of something with an FC on the end of it.
When you look at a base art like boxing, wrestling, judo etc There are intangible qualities, and a style which the fighter uses that will be more suitable for MMA than others equally skilled at that art.
Andre Ward has the ideal style of boxing to transition into MMA, in my opinion. Just like a judoka/BJJ guy whose major techniques don't rely only on gripping the gi.