A syntheist summoner seems like it gives the most benefits to the player who wants to play a summoning PC (you are effectively playing a monster, you can still talk to the pet [from inside it], and you can customize it] with the least annoyance to everyone else. Most customizations would be smite-style spells that are cast on a bonus action, letting the pet use its action/reactions for combat; these can be "class spells" like the mages get (and thus the summoner gets to be a mage). Bigger changes can come from transmutation or abjuration spells from the arcane list (obviously any spell with range: self can apply to the pet, since you will be wearing it).
I could see subclasses being big brute (your pet is large), valor (your pet has a weapon), stealth, and chaos (your pet can temporarily change your body, so it and you could act like an ooze or be one size smaller)