I will admit that Mardu does not have as many legendary creatures as other color combos, but that in no way makes it a weaker deck. In fact, a lot of the strongest card combs revolve around W/B, B/R and even R/W. It's the smaller synergies that make the deck work. For example, can you imagine playing a
Rakdos shred freak turn 2 and then have a
zealous strike ready to spring by turn 3? Though it is just one
simple combo the color scheme revolves around skill IN COMBAT. When it comes to dishing out damage and reducing enemy forces to zero this group has it going on.
As far as what color combos are weakest? That's a touchy subject to go into, simply to put it, everything is weak to something and vice versa. It would be wrong to say that selesnya is the weakest link when it's probably THE most capable of producing small and fast growing armies and having creature recursion, hexproof and even indestructible at its arsenal. Likewise a person can say that B/R or U/R or even Grixis (B/U/R) are the best merely because of the destruction and control it can implement on the opponent. However there are always drawbacks when playing with black and control can lose out to most well built mid ranged and aggro decks. I think that when you ask "What decks are the strongest" you should give a category so as to narrow down on what you mean.
To answer the question I think that Mardu would be the strongest as far as actual combat and deck stability goes. The White and Black
balance each other out and the red gives the deck direction. This type of deck may be overlooked in large part due to, as WWolfe stated, having the least commanders to choose from.
Oros the avenger is my only commander for that color scheme but I can easily build a mardu deck to last just by having him.