
Mighty Squash |

The Samsaran ability works much better at pillaging from the summoner list than it does at adding to it, especially as summoners have such a limited amount of spells known (and the ability doesn't change that).
Look at what you want to be able to do and if Summoners don't have a spell to do that then work from there - just remember that you'll be limiting your access to summoner spells for every non-summoner spell you take. That said, the bard list is probably the best place to look, then the witch list. The wizard/sorcerer or magus lists will have less to offer unless you are wanting some blasting spells - which seems a bit of a waste of the action economy for a synthesist.