You can choose between "Feast on fear" and "Devil's due". It depends on the map you play. If it's a map where you think you can charge a lot of enemies into the walls you should choose "Feast on fear". The same is for big maps.
But on small maps and if you don't think you can charge a lot you should sein "Devil's due" to get that big heal from pools and globes.
Level 4:

Pick "Souls to the flame". The whole build is around the firestomp so you need to boost it. This talent will grant you souls for hitting enemy heroes with your firestomp and also, you get souls which you need to get bulky.
Level 7:

This is the most important talent. "Eternal flames" will reset your firestomp for every hero to get stunned by your abilities. Not good enough? You also get 35 mana back.

Level 10:

The heroic is not that important but I usually go with "Apocalypse" to split the enemy heroes while we are teamfighting. Also, you can use it to achieve incredible combos with "Apocalypse" -> "Shadow charge" -> "Overpower". If you're very quick you can put some firestomps between the abilities ro reset them, get more damage and heal.

Level 13:

At this point, most talents depend on your personal opinion. I choose "Devastating Charge" to lower the armor of my enemies and give my team the best chances to kill them. But if you want to do more damage by yourself you can choose "Hellfire" to get Spell power for every hero you hit with "Firestomp".

Level 16:

Normally I choose "Domination" to extend my stunchain and get more resets of "Firestomp". But you can choose "Debilitating flames" to slow enemy heroes. For every flame of "Firestomp" which hits an enemy, this enemy gets slowed for 5%. At 25% this enemy will deal 50% less damage.

Level 20:

At level 20 there is no discussion. "Hellgate" is just the best talent because it gives Diablo a teleport with a mini apocalypse. This comes in handy if you mastered the stunchain of Diablo and the combo of his abilities.