I don't mind the Ultramarines per se. What I mind is that they're flawless. They don't have any baggage over ten thousand years, just a long list of (relatively) good things they've done in the name of the Emperor. Most Chapters have something they sort of rock their heels and whistle innocently about when someone asks, but not the Ultramarines. It makes them dull. They're like Superman without the kryptonite and racist propaganda.
I think a lot of the hate for the teeming masses of Ultramarines comes from the fact that people new to the hobby see them on all the covers for the boxes and assume that's what all Space Marines are supposed to look like. I know I did when I got into 40k ten years ago, when the Emperor still walked amongst his people... and a lot of folks just don't bother changing to a different Chapter when they realize that there are just so many cooler things out there, and their models don't have to be blue.
Mercifully, in Indiana, people seem to tend to shy away from the Ultramarines, because if you show up at a store with them, everyone's first reaction is, "Oh, you're a Colts fan!" Now, there's nothing wrong with the Colts (they're one of the top teams at the moment, after all) but it definitely takes away from the models all the same for some reason. When I painted the store's Assault on Black Reach display, I was going to do Ultramarines, but I had visions of passersby looking into the window and saying, "Oh, look, the Colts are fighting those green monsters!" The store gets enough traffic from random people due to its location that I decided to go with a different Chapter-- and the Blood Angels fighting Orks is a classic matchup all the same, so it works.
The real reason I think Ultramarines are the flagship Chapter, though, is because they're really god damned easy to paint. It's outrageously difficult to screw up blue power armour, so if you're new to the hobby end of things, you can make a presentable army without any time-honed skill. There's something to be said for that, too-- it helps keep people from getting discouraged.
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