Brown fur transmuter arcanist archetype is probably the best pure caster for it, raw numbers wise it makes the stat bonuses from polymorphing even bigger, and for fun you can also turn your party fighter into a whatever.
Bloodrager kind of wins the day as far as a framework for stacking polymorph buffs on though. Problem is its later in the career and usually limited (dragon form or elemental form) in scope.
Alchemists get polymorph spells, some of them at spell levels reduced enough you get them at the same character level as a full spellcaster. They're noticeably better at fighting than sorc/wiz/arcanist before you apply buffs, if not as good as a bloodrager.
An investigator is similar past the lowest levels (1-3) where they lack combat ability.
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Sorcerer is my pick (but Oracles can be better if abused properly). I enjoy sorcerer for when I take Dragon Disciple levels. The question is a bit more convoluted than it appears at first blush, too. Each form spell is mostly the same but if you have a preferred creature form or ability the DC’s tend to be based on the spells DC. In this way oracles and sorcerers are smart plays because you can leverage charisma in myriad ways beyond just your casting. However, if you are looking at poison I think one or two of the alchemist archetypes are simply better at it as the discoveries and archetype abilities can make the venom more powerful and versatile than a spells granted version.
Pure caster ‘transformers’ are actually extremely competent combatants. You typically build for strength and/or Dex followed by your casting stat. I’ll second the shapechanger bloodline. That power is incredibly potent and sexy and it’s almost exactly what everyone wants when they change shape. I attempted to build an npc modeled after Morgan’s mother in the original Dragon Age. If you ultimately disobey her and fight her, she reveals her true form and it’s a fun and daunting experience.
Oracles with the proper mysteries can do this with hour per level durations as revelations but their forms are limited.
Make sure to select gear that functions even when you are polymorphed and you will likely want to rely on some specific buffs based on the impact of your new form.
Also, depending on how many natural attacks your form has and whether or not you are changing into forms for melee combat or not, but Multiattack is another feat to consider later on if/when you have a high enough attack bonus and number of natural attacks. I suspect there is a wildshaping Druid guise with relevant information on building for combat. I would hunt one down and see if that helps you.