Okay, there are two kinds of mental cheating that I'd like to point to. The first is the involuntary 'falling in love' with another person while already in a relationship, but you still love your current partner. This has the potential for the most harm, because everyone involved is now incredibly vulnerable, and given the heavy emphasis on monogomous relationships in our respective societies, it is almost impossible for someone not to get hurt. The cheater will lose someone they love either way, and have to deal with the thought that they are betraying both of the people they love by loving the other, the original partner will have to deal with a sense of betrayal from the cheater, regardless of how it turns out (or most people will anyway), and the non-partner, assuming they love the cheater back, will cause the cheater emotional harm no matter how it turns out, which if they love each other, will be painful for the non-partner too. The second kind of mental cheating would be where the cheater loses interest in their current partner while developing interest in the non-partner. This is worse, even though less people are hurt. It is worse because, unlike the other one, where the cheater is as much a victim as anyone else, in this scenario the original partner is being strung along, and when they find out, it won't be a giant ball of sympathetic pain, it will be outright betrayal, with no compassion or empathy. It is the metaphorical ripping out of the original partner's heart, putting it on the ground and grinding it into the dirt with the heel of the cheater's foot.
Physical cheating is either a huge mistake (drunken accident, moment of weakness, whatever you want to call it), or it is the second kind of mental cheating, except they don't even maintain the illusion of a monogomous relationship.
Okay, so in order from worst to ... least worse, I'd go physical Cheating-2, Mental cheating-2, Mental cheating-1, Physical cheating-1.