This answer is intriguing and I'm curious if it's correct, and especially which effects are most important.
If you want to avoid gravity assists, the most fuel-efficient way out of the Solar System is to launch due East from from a launch site in the Ecuadorean Andes, sometime before local midnight on a July 4 when there's a new moon. This gives you the maximum possible benefit from the Earth's movement, leaving only about 12,000 m/s of delta-V needed in excess of Earth escape velocity.
Breaking that down:
- launch due East
- site in the Ecuadorean Andes
- sometime before local midnight
- on a July 4
- when there's a new moon
How do these rank in terms of contribution towards lowering delta-V to escape velocity from the Solar system? Do numbers 4 and 5 have more to do with Earth's instantaneous velocity, or due to a reduction of the gravitational well?