Storm. my reasons are as follows:
We' ll start with Deffense:
#1
As to the speed of Namor's reflexes I won't argue (not that familiar).
However, it is canon that Storm can see the impulses (neurons firing in the brian) from thought to muscle to action. This gives her the edge in any fight (as stated in canon) Think Nedji's "Beyakugon".
#2
As for her speed, Mach 1 at best (utilizing a fast moving jet stream). Storm has also been shown, twice, traveling by lightning
#3
As per her duribility, nothing has ever been shown to pierce/puncture her "Jovian Preasure field" (a field capable of busting through mountains, while carying IceMan and Colossus). If she encased herself in a preasure field, Namor would have a hard time getting to her.
#4
Storm can create any number of atmospheric stalling/confusing phenomina (sp)
Fog, Dense Cloud Cover, Sand Storms, Sun blinding (via the same process used to create the "Sun Lense". and would probably take less effort and time), Heat, and Cold Extremes.
Now, for Offense:
#1 Lightning (Hand or environment generated). As stated before, at it's best is 3x's hotter than the surface of the sun. (Namor can absorb and rechannel energy, but to what degree? Is he a better candidate than Polaris who was overwheled by Storm's power?)
#2 Plasma (Hand generated). Hotter than lightning (think Lightning beam)
#3 Electrolyisis (sp) (Hand generated). Breaks water down to it's base componants of Hydrogen and oxygen.
#4 Winds (hand and environment generated): up Drafts, Down Drafts, Jet Stream, Gale Force, Hurricanes, Tornedoes, Typhoons, Jovian preasure fields, Down Bursts, Fog, Pollution (concentrated into a poison gas), and by extension Acid Rain.
#5 Flash Freezing (environment generated) (credits include: Colossus, Omega Red, Phalanx)
#6 Hail (environment generated)
#7 Sweltering Heat (environment generated)
#8 Sun Beam (environment generated)
If all these weapons don't give her the edge, Then the fact that she can combine any # of these in infinite different ways most asuredly does.
Also consider the fact that Storm's powers are Omindirectional. That being said, the longer the fight lasts the more Namor will weaken (out of the water, and under EXTREME environmental duress) Storm's odds increase dramatically, if not exponentially.