Most regular snaffles (single jointed, double jointed, mullen; loose rings, D-ring, eggbutt, etc.) are about the same in terms of harshness, but some horses have strong preferences about which they prefer while others don't seem to care.
There are a lot of harsh bits out there. My least favorite commonly found bits are double twisted wire snaffles. There's just no way to communicate with the horse without causing pain in those.
Mullen mouth is a bit with no joints- it's just a solid bar. They're usually pretty mild, but could potentially be harsh with certain cheekpieces or if very thin.
Rubber covered bits can be milder, but are generally very thick, which can be uncomfortable for some horses, particularly those without much space in their mouths.
I use a KK-Ultra 2-Type
for dressage (essentially a double jointed loose ring snaffle), and a double jointed eggbutt with copper roller
for jumping. I use different bits for different activities at the suggestion of my trainer- not because one is better suited per se (although the copper roller is not legal for showing in dressage)