They are both geniuses, can't pick either one. Love the stuff both of them have done, Mikael with Opeth and Dan with EoS, Nightingale, Bloodbath, Diabolical Masquerade and not least his solo album Moontower which is awesome.
Both are very versatile also, Dan maybe a bit more with his stunning producing/arranging/mixing ears and his numerous instrument playing skills (decent guitarist, bassist and keyboardist aside from his marvellous vocals and drum skills) but when it comes to composing music and especially writing lyrics I have to give a slight edge to Mikael.
But as both are really great musicians/composers/etc as well as unique and different, it's impossible for me to pick one above the other. Lucky I don't have to!