Ultrarealistic, cartoony, 3D, 2D, prerendered, pixel art I like them all, as long as it's tastefully done. Some genres lend themselves better to 2D or 3D, for example old school Final Fight / Streets of Rage brawlers just look bad with the characters in 3D, its best done in 2D or a hybrid with 3D world and 2D sprites.
Anyone who likes colour should check out CAVEs (shmups, iphone games, Nin2Jump), they are masters at the use of vibrant colour and making colours pop out. Much of it is borne from the constraints of the bullet hell / shmup genre (muted backgrounds and vibrant projectiles to aid in reaction times)
Sigh... I would show a few examples however this forum is sadly lacking in img linking and I don't do HTML. I would suggest ppl google image search these keywords:-
Mushihimisama Futari, Dodonpachi Resurrection Black label, Deathsmiles, Akia Katana, Nin2Jump, Esp Galuda 2, Guwange.
EDIT heres a 720p vid best to watch it in full screen imo.
Also the platformer Outlander does a similar thing,the dev like CAVE makes shmups where they have converted some of the artstyle and gameplay to a platformer
I also like pixelart however it really need a old CRT to look at them, looking at them on a LCD is ok but it will always compare badly to HD computer art.
* Derek Yu the man behind Aquaria and Splunkey XBLA makes some great looking 2D games, the guy is a trained illustrator BTW. Konjak aka Noitu Love 2 is also a talented 2D game artist who also freelances with Working Designs, Contra 4, Mighty Switch Force etc.