In a vacuum, What do you think is the best Unchained Eidolon Subtype (e.g Aberation) for a Soulbound Summoner?
When taking into consideration the archetype rule:
At 7th level, the eidolon can select evolutions even if it does not meet the subtype requirements for that evolution.
While not necessarily the most powerful or optimized I think combining the Soulbound and Twinned archetypes is thematically interesting. The combination allows for the eidolon to do some limited spellcasting and if you build toward the Psychic Magic evolutions available only to aberrant subtype you can make an interesting pseudo caster eidolon.
I'd probably go for elemental. It starts out with strong immunities (chosen element, sleep, paralysis), can take any base form (well, not aberrant, but quadruped unlocks pounce) and has access to most evolutions (you miss out on tentacles and web, before level 7 in your case).
Agathion subtype, hands (literally) down. It gets Lay on Hands as a Paladin equal to its level once you reach level 8. There is a good bit of stuff you can build around that to make a fairly powerful critter. But, that is just my opinion on what is 'best', rather than what is the most powerful. Other's might think that power = best.