Barbarian is the best class for a supporting character without a subclass.
- Most hit points (1d12 hit die).
- Unarmored defence (Start lvl 1 with 18 AC, 16 Dex[+3], 16 Con[+3] and a shield[+2].
- Extra attack at level 5
- Fast movement (10 foot speed increase at level 5).
- Rage - you still get this classes defining feature at level 1 without a subclass.
Compare Ranger which is completely nerfed without a subclass. Barring colossus slayer / animal companion and Ranger loses it’s soul.
Hot take: The most important aspect of a supporting character is their staying alive. Because the most annoying would be having to constantly revive a nerfed PC. So Wizards are out, rangers and warlocks and sorcerers are out, your left with cleric, paladin, fighter, druid. Well fighters are okay with second wind and action surge but its a debatable second fiddle to Barbarian hit die and fast movement. Losing a druidic circle is harsh but workable. Cleric is fine but boring, party healer if your into that. Paladin flavour-wise could be a headache keeping their oath etc.Barbarian won’t die, is easy to RP and doesn’t-really lose anything of import when compared to other classes.
Subjective support: I have played D&D, read PHB.