I'd say it's the vanilla Summoner. He's just an all-around great class, such that any archetype is going to be trading good things and bad things. That's not to say other archetypes are bad, but the baseline is so high that none of them are going to exceed it except in specific or narrow ways.
The vanilla Summoner is just a really good jack of all trades option, with other archetypes like the Synthesist bringing in pretty hefty damage outputs. Either way, both are excellent options.