Because Ragnaros gets really bad players, he often underperforms - but not due to him being a weak hero. Therefore I decided to throw few tips here. To clarify, I am plat/diax player on EU and Ragnaros is one of my favourite heroes (tbh nowadays I play Stukov/Ragnaros/Zul’jin if I can pick one of these, due to their impact on the game). I reach 60-70% win rate with Ragnaros on draft, so I think it is fair from me to say I am at least decent 
1. General playstyle and drafting
Ragnaros is a melee damager that performs best with multiple tank/bruiser compositions (ie. 1 tank 1 bruiser OR 2 tank OR 2 bruiser), since he is quite squishy and vulnerable to crowd control. Most times he should wait for another character to engage OR if he can do that safely, use his trait if enemies are dumb enough to get in the trait Q range, as Q+W+sulfuras smash+E if cast fast enough can delte most damager and healer heroes.
Great power of Ragnaros is that he can clear waves fast, often allowing him to roam between 2 lanes and soak both of them, especially when his 4 man is pushing top or bot lane, leaving enemies a dillema: either get punished by 4 man or getting someone to either depush Rag or kill him, which is not exactly an easy task for most heroes.
Do not pick lava wave vs zerg on Braxis, it deals nearly none damage nowadays (it was nerfed specially for this map since Rag was too op on it). If you can hit sulfuras smash, dont pick lava wave. it is nearly always better to have huge damage + stun ult that has quite short CD than to pick a lava wave that will destroy minions and damage mercs a bit (and maybe some noobs if you happen to play vs them)
2. Different skills and how to use them
Q is one of the strongest spells in game when picked at lvl 1, 7, 13 and 16. It grants you insane aoe damage with the build and if you manage to hit 2 heroes with it, it returns half cooldown and mana. Basic Q has decent healing as well.
Even if you are not using Q build, remember to click Q in moment you see Ragnaros doing auto attack animation. Why? Because Q resets auto attack timer (auto attack speed is just a cooldown and Q resets that). Since Q animation is slower than basic auto, perfectly you’d like to cast Q when animation is already starting, as the auto attack will become Q AND next auto will be nearly instant.
W known as the meteor is a long cooldown spell that, due to recent lvl 7 W buff became a decent build, allowing crazy range damage with relatively short CD to happen - if you hit enemies of course. Since W doesn’t change its direction until upgraded (and even if upgraded, it can be done only once) you want to impact enemy movement with it. If they are chasing you, force them to run in the hitbox of W. If they want to escape, cast it such way, that they have to dodge it or lose tons of hp. And if they dodge, you get closer to them.
E is a tricky skill, that should be used differently in different situations (and with E build it is quite different). Base E has quite low aoe damage, but speeds a character significantly, so you can use it for a Garosh to let him catch an enemy OR give it to ally that is trying to escape.
E build however grants this skill another way of use, since it deals decent aoe damage, explodes twice (so speeds you twice too) and shields you for each enemy hero hit, which can be insanely strong if enemies are grouping a lot.
Sulfuras smash ® or as I call it “Mjolnir” is one of the best ultimates in game in my opinion. It has quite low cooldown and if hit well, its centre will secure a kill or make ETC regret using his mosh pit. It can be dodged by a mobile character easily (unless you time it well in Tracer’s recall so she returns exactly into the epicentre), but heroes with low mobility or on CD have hard time with it. It also allows you to solo most of characters that would normally beat you 1v1.
Lava wave ® - sieging ult, that kills minions instantly and sometimes can kill noobs and hurt mercs a bit. What you need to know about it, is that if enemy tries to escape from it by retreating to their nexus direction, they get massive damage. HOWEVER, if they decide to run straight into it, they receive minimal damage below 10% of their hp (unless they are hella squishy). It has long cooldown and is nearly useless at teamfights (or totally useless at teamfights) and grants Ragnaros possibility of clearing waves, which could perhaps be nice if he wouldn’t be able to clear two lanes at same time with basic skills just by roaming between them.
Trait (D) - it can be used on allied fort/keep or on “skeleton” of any fort/keep, no matter was it yours or enemy’s. It is actually quite decent due to stun on Q and large range and aoe damage. You can also cast ultimate ® when using trait which is good to remember.
You shouldn’t use your trait to tank objectives or bosses most times, cause it gets deleted nearly instantly and gets no value. It is very good at finishing off enemies or stopping them from diving for kills under structures. Ideally, you should use quickcast/fast use/whatever you call it for Q, E and R and hold the line damage skill of trait (W) on normal cast.
Trait Q needs a bit of time to activate, but is huge aoe stun that allows you to cast all your spells and ult on stunned enemies.
Trait W and E are used mostly to damage enemies.
Lvl 20 stasis - must have pick most games, as it allows you not only to become unkillable for a moment, but you cooldowns are still counting AND you are healing yourself for a decent ammount of HP. Use it wisely!
3. Builds
Q build:
Lvl 1 Q
Lvl 4 globe stacking
Lvl 7 Q
Lvl 10 Sulfuras smash
Lvl 13 Q
Lvl 16 Q
Lvl 20 stasis
This build requires you to hit your Q’s on multiple enemies at once and time it well with auto attacks in order to deal great number of damage. You also need to stack on minions early game which can be a pain with dumbass teammates not allowing you to lost hit minions, but even then, you should have it stacked before 8th minute (I usually finish it before 3:20 with my team paying attention to me last hitting or before 6:00 with them trolling me XD )
E build
- E
- E
- Auto attack
- Sulfuras
- E
- E
- Stasis
I personally perceive E build as most noobish one, due to low skill needed to use it. With this build you want to burst down low hp tragets whenever you can do it safely. You also get decent shield value from lvl 16 when enemies group up.
W build
- W
- W (can also use globe gathering if enemies have no decent damage spells, but even then I’d still pick it)
- W
- Sulfuras
- Any you find suitable for the situation
- W
- Stasis
With this build you need to hit heroes in order to increase W damage and reduce its timing. As said before, use it to interrupt enemy’s movement, especially good when someone tries to gank you (and his name is not Genji 
)
4. Counters
Q build is countered by blinds and stuns like BW or Uther. W is not a wise build vs. high skill enemies or vs lots of mobile heroes. All builds suck vs heavy stunlock. Heroes like Sonya or Imperius can solo Ragnaros easily (also good Stukov if he manages to hit stunlocks on you or you lack sulfuras smash atm). Greymane can hurt you, but most times you can outplay him. In general, Ragnaros is one of the most flexible heroes and therefore you don’t have to worry much for enemy teamcomp, but don’t pick him when you miss healer or tank (unless you are going multiple bruiser teamcomp).
5. Maps and level
Two lane maps are bad for you most times, especially when you are forced to solo on Braxis, early game is a nightmare. You are able to do camps on most maps quite fast tho (other than bosses) and if there are 2 lanes close to each other you can soak both at once (Sky Temple, Garden of Terror, Volskaya, Dragon Shrines and so on, these maps are really good for you).
As you gain talents, it changes, as Ragnaros after lvl 7 gets decent values and after 20 is one of the strongest damagers in the game.
