The Ultramarines are way too over represented and to be honest I feel that the Imperial Fists are A) way more cool and B) need more exposure. They also kinda over-lap with their roles in that they are both long-ranged defensive chapters but once again the Ultramarines are not liked by many because they are far too mainstream and boring.
Black Templars would have been my vote; they're pretty damn iconic and the game is called "Eternal Crusade". I agree with you about the Ultramarines, though. They really are the bog-standard "Space Marine". Aside from adhering to the Codex Astartes, they don't really have any specialty or something that makes them unique. Imperial Fists would have been baller. Without Templars I'd play a Fist for sure.
it's like he wrote a thesis on his hate for smurfs. took one word from every paragraph, and then put them all together.
Simply because chapters are already announced and there are no reasons for such a change, there are more ultrasmurfs fans than yellowbrick marines ones. More novels, more artworks, more everything, they are iconic. You need not to love all-shiny-good (and-boring-as-hell) boys-in-blue, just deal with all the facts.