Ragnaros - Default

Level 4

Slow Burn provides a relatively strong slow, especially when chaining with Blast Echo , and is a safe default pick if the other defensive talents on the tier aren't clearly better

Fire Ward provides decent spell armor against infrequent, high damage abilities. DotS will strip stacks, but if the meteor keeps hitting as it rolls it can keep adding new stacks. Consider this if you're getting poked by high damage magic spells like Chromie Q's, and can reliably refresh the stacks in fights.

Catching Fire provides a small amount of HP regen, and on completion gives you fairly spammable armor. This can help trade slightly better, and serves as a nice anti-gank tool for a fairly squishy offlaner. Pick this if you'll be under heavy gank pressure which Fire Ward wouldn't help with.

Level 13

Tempered Flame provides a strong amount of sustain, especially in combination with Blast Echo , making it the strongest raw throughput talent on the tier.

Resilient Flame provides extra armor when you're getting CC chained. Pick this if you're at risk of getting picked off early in fights and the save tools aren't getting to you fast enough. Do not pick this if someone else is providing armor to the entire team.

Level 20

Submerge provides a nice safety net for a relatively squishy teamfighter, letting you dodge key abilities and recoup health.

Lava Surge makes it incredibly hard for opponents to end the game without a team wipe, as every wave will be constantly pushing against them. Pick this if you're not too worried about fights, and are mostly playing to stall out the game.