What is the highest Aniimo grade?

Elite sits one step below Perfect, and an Aniimo counts as Perfect when both of its inherent potential values are 8. The grade sets the stat ceiling, so at the same level and personality a Perfect Aniimo has higher stats than a lower-graded one.

You can't see the grade straight away. A fresh catch shows as "Unappraised" until you submit it in a catch report at an Outpost's Research Console. Once it has been appraised, the storage box can filter by Potential grade (Common, Good, Elite or Perfect), which is the quick way to find your Perfects in a full box.

Rarity is a separate thing. Alpha and Prismana are rarity values, and a Prismana is not guaranteed to be Perfect. Acquired potential is separate too. You train it in the Awaken Potential tab with Stardust, Star Sand and Star Essence, and it does not change the innate grade.

Where a guaranteed Perfect comes from

Perfect Eggs always hatch with perfect potential, whatever item you use on them, and Umbral eggs do the same. The chosen Battle Pass Prismana egg hatches a Perfect-potential Aniimo such as Prismana Formentus. That guarantee is written in the item description but not on the hatching screen.

One step down, Incredible Eggs (also called Elite Eggs, found in places such as Beast Fang Ridge) guarantee Elite or higher, and an Ultra Aniipod sets a minimum of around Elite. The Sparkling Cube is a paid pod with a 100% catch rate, but it has no settled Perfect guarantee. It can still produce stats below Perfect, though it never lowers quality.

Not settled: Perfect is the top grade in every version of the tier list, but the grades below it are not settled. The usual set is Common, Good, Elite and Perfect, while Ordinary, Normal, Rare and Epic also appear as lower names. It is also unsettled whether wild Umbral Aniimo are always Perfect the way Umbral eggs are.