Depends on your own preferences. There are 3 options.
1. If you're a fan of the art, like oversized editions, and don't mind the price, then the Absolute Editions are the best option.
2. If you want just a few volumes for the whole series, then the omnibus format is best.
3. If omnibus format is too bulky/thick, then the Deluxe editions are high quality, but not as thick.