There aren't many A-List level characters out there. Sub-Mariner gets to sit upon his throne (Calling his kingdom "Atlantis" didn't come about until the 1960's.) and sometimes his people are there to rally around him and sometimes they've all turned against him and swum away. For some insane reason, he keeps Krang nearby despite his numerous attempts at overthrow and assassination. At least Maximus and Loki are relatives. Krang should be put up against a wall and shot, or whatever the underwater equivalent of same is. Byrrah's a treacherous relative, but maybe Marvel has played this card a little too often for Byrrah to work well anymore. 

Sub-Mariner's ties to the FF hamper his status these days since the FF aren't in a good place (or anyplace) at the moment. The Inhumans are bogged down in "royalty" and "greater responsibilities," in much the same way that Namor often is. That doesn't contain a lot of intrinsic appeal for a modern-day character. We'll watch "Game of Thrones" and ooh and ahh at dragons and fantasy settings, but a modern monarchy, especially one populated with stock characters as Namor's is, doesn't inspire reader loyalty. Could he overcome his obstacles and make it to the top? I think he could, but he's not there now. 

He's a classic character, but sadly, not an A-List one.

Loved that series as a kid.

But Brian does raise a very good point that I had not thought of: which is, the quality of the supporting cast. I think that is one good measure of an A-lister (among others).

Now, I dunno. I suppose he has sort of muffled around the MU as a supporting character for far longer than those stints where he has played the lead, but I think he is just waiting to regain that A-list status.

When JB was telling monthly tales with Namor, it felt very right.