Interesting question. Now that i think about it i can't think of a ship that would immediately come to mind.
The ikaruga ship looks wicked, but most of the details are lost ingame.
Vic viper scores pretty low on my score list as it looks like your standard f16 with some extras attached to it.
I think if you are to design a ship it's most important to fit the rest of the game and have a consistent art style, fitting the theme you have in mind, more than
just have an awesome ship.
I am also struggling to find an art direction for my shmup, but i think i finally got a
push in the right direction.
One thing i just remembered:
http://www.demoscene.tv/prod.php?id_prod=13138 [Slightly NSFW]
This is a quite spectacular demo with some interesting and neat vehicle designs. It was very inspiring to me.
Also, i agree, the R-GRAY looks rather odd. These are some weird proportions :D but hey, it's art, anything goes.