oh boy! after getting through the thread, I have to still say Ozai.
lets go down the list, yes?
first of all, keep in mind Goku's own parenting. he was essentially a semi-wild child when Dragon ball started, after his care giver Gohan died. even if you ignore the fact his race was known for sending their own children off on planet obliteration missions (circa DBZ), you have to allow for his lack of common social knowledge. being a father wasn't one of them. Being a good PERSON was.
as for his "abandoning" of gohan to piccolo: no. Goku was dead, Piccolo was there, saw the kids power, wanted to train it. NOBODY could have stopped him from doing so, since he was the strongest person left there. Piccolo eventually becomes gohan's second father, yes, but I think the excuse of "I died to help save everyone, INCLUDING my son" gets goku off the hook there.
as for the other times by series Gohan was abandoned by Goku, (that i remember) we have the end of the namek saga, and if you count it, the ending of the cell and majin sagas. no, we do not bother to count GT.
admitted, the majin saga is a ??? moment, and the namek saga ending almost caused trouble, but even then Goku had instant trasmission and was intending to stop the starting events when trunks stepped in. but the Cell saga I got. remember, ever since DBZ started every villain character that came to earth was to seek Goku out. raditz, Nappa, Vegeta,Freeza (after his saga ended), and ALL of the androids including cell all had a bone to pick with goku. he figured that if he stayed around he'd cause even more trouble on earth, so he stayed dead at the end of the cell saga thinking it would save everyone trouble. at least, until executive meddling and the majin into GT...
Gohan went to Namek to save piccolo who he respected, and when Goku did have time with Gohan he still wasn't a bad father in any respect. if memory serves Goku and Chichi also got along well despite all the mess they had, and goku did agree to let gohan become a scholar once all the mess of the androids was averted. if occasional abandonment's is all there is on his list, considering he may as well be the Japanese superman for the amount of punishment he took and lives he tried to save, then Ozai... Ozai wins.
Goku may be an idiot at times, but his hearts in the right place. Ozai? oh boy, lets see...
multiple attempts to kill Zuko, banishing Zuko for backtalk, scarring his face and forcing him to go hunt for a person that's been missing for 100 years... and yet whenever Zuko has the chance in the early seasons, he tried to get back in his fathers good graces. never mind what happened to his mother so Ozai could get on the throne in the first place.
Ozai was a maniac who used his kids as chips to get what he wanted. he prepped Azula into what we first saw her as, and she turned out to be JUST a trip herself. She threatened and manipulated people, and would have killed Zuko to, but she knew how much he wanted to be back in dear ol dads good graces, and used that almost as often as fire bending on him. when all that back-drafted on her.. it literally gave me chills. yeah. while you can argue Goten didn't get anything from Goku, I'm pretty sure we've got Ozais full range of parenting issues at hand here.
the kid he hates gets scarred for life and multiple murder attempts put on his head, and the one he favors goes bonkers. that's 0 for 2, Ozai. at least when Goku put Gohan in harms way, he had a reason of proving gohan was stronger then him at the time, and was able to save the world, despite the craziness of the move.
People here are giving Ozai a pass because he was already a bad guy. y'know what? Goku may not be a good father by any stretch, but ill take a self-sacrificing father that keeps the planet I live on from destruction even if i don't get to see him, versus being beaten around by a king who doesn't want anything to do with me unless I can be a political chip somehow. oh, and intended to incinerate everyone to fulfill his aims of global conquest.
But really, both fathers are busy busy folks. still though, even though Ozai wins this round, he would lose to Charles Zi Britannia (Ozai 3.0) and Gendo Ikari (mindscrew case #1) in this one.
ho boy. addenda (TLDR): Ozai by landslide, even accounting for Goku's parental stupidity. remember, who'd you'd rather have, an abusive dad, or a well meaning one that has to leave for years to do things that are for the good of everyone else?
still, neither compared to Gendo Ikari or Charles zi Britannia.