Dragon: Bug makes cool attack not occur and battle easy if you can clutch, egg and gateway reward. 4/10 risk 8/10 reward.
Wither: Easy to cheese with just one iron golem, some ingots to fix it, and a deepslate tunnel, beacon star reward. 1/10 risk 8/10 reward.
Warden: Not even a boss and easy to cheese (25 block pillar+pwr5/dripstone), but can pack a punch if not cheesed, reward: one catalyst.9/10 risk 1/10 reward.
Elder guardian: Damaging, tanky, interesting mechanics (laser, thorns, fatigue), three of them, sponge and farm location as reward. 7/10 risk 7/10 reward.
Edit: warden reward
