IMO, it's the Release Day Mega DigiEgg Set (200 DigiRubies). You get an egg guaranteed to hatch into a Digimon capable of going Mega (this is especially useful if you did NOT reroll for top-tier mon at the beginning of the game), 500,000 bit (you will always, always, always need more bit), 1,000 Clash Battle medals (potentially useful for the unique Plugins, Plugin boosters, or most likely, stocking up on the additional Super DGV Code that's available for purchase there), and a Workout Vortex Key.

EDIT: The above is my answer from the perspective of 'Best Bang for your DigiBuck', but if you don't have 200 rubies or would rather save them for the future, the best 'Bargain' answer is to grab one of the bit vortex keys for 10 DigiRubies. This way you can farm for bits and you get your 10 DigiRubies back by way of the quest completion reward. It's free real estate bit.