General Mattis is a soft spoken and very intelligent Marine. He is a fellow at the Hoover Institute. There are some videos on youtube of some sessions he has done. Well worth the watch and showcases his knowledge of foreign policy.
As for the current candidates, governors and senators are out automatically. The backlash against mainstream government and media is growing. Only an outsider will have a chance.
Trump didn't exactly come from nowhere, he debated running 4 years ago, and had quite a lot of support then. He appeals to the masses under the old 'the enemy of my enemy is my friend'. The more officials and media trash him, the more polarized his support becomes, and he doesn't give 2 shits about donors or what anyone says about him. I wouldn't be surprised if he won the nomination. Just my 2c.