Incinerate is the strongest and most useful destruction spell for the end game. Seventy-five points in Destruction gives you Incinerate which is usable once you have free Destruction spells from -25% Destruction Spell Cost enchants. Also, b e aware that leveling unnecessary skills will increase the levels of the enemies that you will be fighting as well. The Breton race is recommended for all builds on legendary difficulty due to the importance of magicka resist.
Fire is the best option because it has the most damage for cost of the spell compared to other magics. Incinerate dual wielded will be your end game spell. Firebolt and Fireball, as you level, are best used for ranged casts when you can't reach the enemy as they can be pretty expensive for spells. In fact you can cast it for a split second every once in a while and still get full damage without having to channel it which costs a lot more magicka.
Flames damage is over one second so don't feel you need to hold down flames for damage. Flames will be your go to spell early on and you can also dual wield it with both hands for even more damage.
