Fair enough
Anyways, Gold Mask is only really my choice because he's a 1-5 boss, and at that point you don't have many buffs and probably don't have that good of a weapon. There are some 3-5 bosses that are up there, too. Probably shouldn't have said "by far". Zulan can be hard, but is generally fine if you have a good bullet clearer. Dark Grand Knight (yes I know he's a 2-5 boss) is designed to be a nightmare, but gets memed by fast melee weapons and the reflect buff. FLUFO is the same, but is generally more bearable without one. Vark leader can be a huge pain, but if you have the poison buff and either a decent bullet clearer or decent dodging skills, he's fine.
Baby dragon bros and sandworm are a different story. Baby dragon bros are a duo boss designed like two single bosses mashed together. They have little to no telegraphing for their attacks, which include a large sweeping laser for both that they can use at the same time, requiring either luck or iframes to avoid. The other attacks are made hard to dodge by the other dragon's current attack or are near-impossible to see. Sandworm is perfectly fine until he starts digging. When underground, you can't do anything and it's hard to tell where he is, making you easy prey for the unavoidable 3 contact damage when he comes back up. The digging is made worse by the bullets he shoots every time he comes up and then dives down again. Worst of all, how long the phase lasts is entirely up to rng, and even when it does end, you are immediately met with a blast of bouncing bullets. The digging attack alone makes this fight a huge pain.
But at this point you are much better equipped than at 1-5, and that's what keeps gold mask at the top.