"You were wrong, Thor, it Never Ends...and Each Trial is Harder than the last!"
Wanda Maximoff - Earth 616
I'm going to say Namor is his own worst enemy. The man's to proud to admit he can be wrong or need help.
I'd say Llyra.
She killed Namor's wife Dorma.
Most of Namor's best known foes then to be of the palace intrigue variety (Attuma, Krang, Byrrya, Llyra) who can't hang with Namor in a straight fight.
The foes that could match with Namor physically (Orca, Tiger Shark, Invaders foe U-Man, Avengers baddie Tyrak ) we don't see that much.
"...Doom's enemies have not the mettle to challenge him host to host, tooth to nail... As economic and military options fail them, they resort to simple rudeness."
Give credit to Jeff Parker for bringing him back and making a significantly creepy improvement to him.
1. Namor. His his own worst enemy. He has lost his temper and gotten into many senseless brawls with other heroes.
2. Iron Man. In terms of sheer quantity of fights, Namor has fought Iron Man more than anybody else.
3. Hulk. Namor has also clashed with Hulk on many occasions.
4. Llyra. Killed Namor's wife on their wedding day.
5. Doctor Dorcas. Killed Namor's old girlfriend right in front of him.
6. Doctor Doom: Namor's best frenemy.
7./8. tie: Attuma or Krang, they've both clashed with Namor several times over the years.
9. Tiger Shark. He is roughly just as powerful as Namor, except that he can't fly.
10. Reed, Ben and Johnny. Namor used to throw down against them on a regular basis.
"Injustice anywhere is a threat to justice everywhere."
Martin Luther King Jr., Letter from Birmingham Jail, April 16, 1963