Ring IQ- You just don’t win fights without this being high. Everything seems to revolve around it. 10-14 will win you matches against cans but not against quality. If you want a belt this should be 15 minimum. Most champions need 18-20. 
Chin- The second most important attribute. I’ll take a fighter with 20 Ring IQ and 20 Chin then let the chips fall where they may for the other attributes. 20 chin is granite. 14 would be the recommended lowest for championship material and even then you may need backup. The in fight strategy Jab is effective in reducing KOs while still fighting for the win but eats energy.
Take punishment + Defence - These two seem to run hand in hand. They are the basis of keeping away the TKO from knocking hard at your fighter’s door. I’ve been flashed tkoed with 9 take punishment /12 defence. If these numbers are low watch out for the aggressive brawlers. Both attributes should be a minimum of 12 to feel safe from the flash flurry of fists. I would recommend one of two 15-20 for belt fights. between them.
Accuracy + Aggression - BOMBS AWAY! This is the offensive combo that strikes fear into the hearts of all fighters. These are out of control risk it all I’m going to kill you first berserkers that can end you in a round if not careful. Accuracy is also good for control boxers too, running up the points in a round. Aggression is not necessary to win a belt but should be 18-20 if used as a weapon in a fight. This attribute combined with the pressure in fight tactic can run up a huge round but eats up a ton of energy. High accuracy helps to win fights and I would recommend 14 - 20.
On the canvas- situational. If your fighter lacks a chin this can really save you. If your fighter has 20 chin this is a wasted stat really. Recommend 18-20 with a weak chin.
Stamina- That little bar that reduces down to zero as you fight? Yeah, this guy influences that. This is a forgive me/forgive me not attribute. High stamina will help slow the downward spiral of that bar. Good for bad rounds and especially good for aggressive fighters.
Power- I find power to be an options attribute. It can give you options. Obviously against weak chins it’s an absolute terror stat that gets neutered as your opponent’s chin gets stronger. IMO it is just another potential weapon in your arsenal of ways to win, but not a game changer. When combined with high accuracy it makes the plant your feet in game strategy viable. That said, fear your opponents power. Flash KOs are disheartening to say the least.
The rest of the attributes are… whatever they are and I deal with it. When the time comes to use your experience points to raise a stat, all of the above are most important.