This old chestnut. Anyway:
Summoner (all, but Synthesist drops away),
Not my experience with Synthesis Summoner. For sure they do not have the massive action economy advantages that other summoners get. If that is your only criteria then OK go with that. But they are a serious 2/3 caster that tanks very well, rather than tries to hide. The shear flexibility of the build is best shown with the level 2 spell Lesser Evolution Surge, with this spell they can choose any feat they qualify for, including Expanded Arcana for any spell on their list, and also from all the evolutions for example eg immunity to an energy type, +8 to any skill, or movement/sense/attack powers etc etc. Further it is possible to get regular summoned creatures out then 'tank up' via a buffed Summon Eidolon. It is very different from the other Summoners but inferior or less flexible? I don't think so.