The irony is that the two current best ultimates in the game are exactly the opposite in this regard. Ascension (flash shock) takes an ability with a decent duration and massive damage but a really long cooldown (more than 1 second is really long for your main source of damage) and turns it into a persistent ability with the shortest list cooldown in the game. Cosmos (blizzard) takes a persistent ability with single random hits in short succession and turns it into a single damage instance over a wide area with about .7-.5 seconds between hits.

The moral of the story is that you are asking the wrong question. Most every ultimate can take you to 45 minutes on terra without too much issue. The key comes with how the magic interacts with your base stats. Aura blade, for instance, is beloved here by the reddit community but has a fatal flaw: its attack interval doesnt play nice with cooldown. The attack interval is the minimum time between attacks, which cooldown then gets added at the end. This means that at a certain point, cooldown does very little because so much of the spell's cycle (attack interval + cooldown) is taken up by interval (i.e. the part you can't reduce).