That's a grea [idea. I will definitely consider it.

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

The ender dragon and wither are the only actual bosses on this list, and between those two obviously the dragon blows the wither out of the water.
Elder guardians feel more like elite enemies to me, and the warden isn't even meant to be fought

None of the bosses are particularly enjoyable. I don't think boss fights are Minecraft's strongsuit, really. Maybe it's just me, but they just aren't exciting at all. The lack of special music, the lack of variety, it just makes any boss encounter painfully slow. I guess if I had to pick a boss, it would be the Ender Dragon, mainly because it's the last enemy you fight before you beat the game. I suppose that makes the fight a little more exciting?