Ragnaros is a great melee assasin with a lot of sustein in lane, and a lot of burst damage, while at the same time he can be one of the greatest deffensive heroes ! BOTH BUILDS ARE MADE to support the whole mindset behind the Ragnaros as a melee assasin at both it's aggressiveness and defensiveness !

The choices of 13 ~
At 13 the talent Tempered Flame (cntrl+W= choice) is for more aggresive plays where you intend to "soak some dmg and reposition yourself. Of course for this to happen you will need an enemy draft that will not be able to stomp you down as if u were a thrall.
On the other hand, if they can stomp you down there you have the second option of Resilient Flame (cntrl+E= choice) which will grant you and incredible amount of resistance which will buy you time to keep up with playmaking as a melee hero.

The choices of 20 ~
At the level 20 talent choice we have the same mindset for both of the options here!
By choosing the talent Flames of Sulfuron (cntrl+q=choice), you support your aggressivess but when you can be the guy to pressure the momentum and you have to find a way to survive, it is almost as you HAVE to choose the talent Heroic Diffculty (cntrl+e=choice)and increase your teams deffenses !

p.s. cntrl+q cntrl+w cntrl+e cntrl+r cntrl+f is how i liket o use the hotkeys on fast talent choices!