I might be a liiiittle biased here since Linoone (well, Zigzagoon) was my first ever Shiny, but it really does look nice. The orange and tan kinda makes it look coppery, and the yellow eyes are perfect.
Smeargle's Stadium 2 Shiny form gets an honorable mention. I can't find any pictures to show it off, but it's light grey with dark blue-grey stripes/ears, and a bright aqua paw print and tail tip. I could never figure out why some of Stadium 2's Shinies were so different, but at least some of them were cool.