If you have a PS3, get the PSN store version of Odin Sphere in the PS2 section. The framerate issues are a lot better as it plays from the HDD. Alternatively, if you manage to find the disc version for a reasonable price (but should probably be more expensive than PSN) and have a decent PC it's legal to use that disc on a PS2 emulator and jack up the resolution to make it look even more beautiful and run perfect.
That aside, if you have to pick one game it depends on what you're into/how you're playing. Dragon's Crown is probably their best game, but is probably a lesser experience to Odin Sphere if you're playing by yourself. DC shines with some friends/girlfriend/wife on the couch or friends you know online who have it. I personally would put GrimGrimiore in last place, though that's no slight against the game on it's own terms. It's probably because I'm not as much into the RTS/strategy thing, if you like Ogre Tactics or Final Fantasy Tactics, Disgaea, etc. then it should be your first pick if you'd be playing Dragon's Crown alone.
If you have a Vita, definitely get Muramasa on there as opposed to the Wii, but I'd say playing Odin Sphere first is more important as I find it to be the better game.
No matter which choice you make, you'll be treated to absolutely gorgeous 2D artwork, like playing a beautiful cartoon with bright and vibrant colors, as well as depth, length, and an air of passion and love for games as a medium. Great studio.
Side note: If you become crazy over them like I have, note that most of the members of Vanillaware used to be in Atlus before leaving to form Vanillaware. Before leaving Atlus the team made a Sega Saturn exclusive, Japanese exclusive game called Princess Crown that's absolutely worth playing. I just...don't think many people have a modified/Japanese Saturn and Japanese literacy in order to enjoy it.
Sorry if I made your choice more confusing as opposed to easy : )