Seasonal weapons have 4 versions: Base, Adept, Heretic Arsenal and Adept Heretic Arsenal

TL;DR: Best of the best is an Adept Weapon with Heretic Arsenal, but chances to get one should be rare.

So there are 4 versions of each seasonal weapon:

• Base version: 1 perk per column, nothing new.
• Adept version: 1 perk per column, different icon and shader and you can insert Adept Mods.
• Heretic Arsenal version: 2 perks per column, different glow, they drop fully masterworked and have Runneth Over as an extra origin trait. You can't insert Adept Mods.
• Adept Heretic Arsenal: 2 perks per column, different icon, shader and glow, they drop fully masterworked and have Runneth Over as an extra origin trait. You can insert Adept Mods.

EDIT 1: Heretic Arsenal version doesn't have different shader, but different glow: from the element of the weapon (base) to purple (heretic).