What is the laziest path to power?

[noob]

practicing invocations is just as hard as getting good at singing.

except

you dont need to be lucky and born near a demon that you could bargain with. so bard is easier than warlock. because its environment and npc independent.

Nowhere does it say you have to practice invocations.
Unlike for skills which have rules(that are optional) that indicates increasing skill ranks takes time.
Furthermore if your bard does not use music you need charisma to use spells and not everybody have charisma.
While warlock does not have stat requirement suggesting that invocations does not needs any sort of understanding: with only ones as stats you could use EB and the invocations just fine.
Also lazyness is not the same thing as ease.
There is things that are easy to do that are not lazy at all because it takes long amounts of time.

As for bards being environment and npc independent I can hardly see you gain levels while in a featureless plain with nobody to interact with.