I'd say same as being a good mum.
Considers the children before everything he does. Plans for their care and welfare at all times. Satisfies himself that when he is not with them they are looked after excellently. When he is with them, looks after them excellently. Has fun and sets boundaries. Gives them a great time. Gets them to bed in time, with clean teeth, every night.
Has a constant set of mental checklists in which he thinks about short and long term. Are they ok now, but also what developmental and social needs do they have coming up, and how can their dad plan to make these happen? Does DS need to learn to ride a bike next holidays? Is it worth seeing the doctor about that stubborn verruca? Should I chat to the teacher about whether DD is getting on ok with her friends, she was a bit weepy last week? Can I do a sticker chart to help DS learn to tell the time?? etc etc.
Plans financially and emotionally for their future. Works hard to keep the family afloat (whether separated or not).
Warms their pyjamas on the radiator.
Listens to them talk. Holds space for them.
Cheerleads them as they take risks, and tells them they are strong and brilliant and resilient (This one is particularly important for dads to do - dad is the first learning for the baby about the world outside mum's arms, and he has to help his child navigate it).
There are so many amazing dads out there x