Soartex, Hi-Res HD Minecraft Texture Pack (64 x 64)
Soartex is the nicest hi-res texture pack I've seen. It avoids many of the issues associated with other hi res HD texture packs in that it doesn't force unnatural (allegedly 'realistic') looking textures into the game and it actually tiles properly. A lot of people end up hating 64 x 64 and higher texture packs because they ruin the general feel of the game and just don't look that great, but Soartex is a texture pack that makes HD worth your while. So even if you've given up on HD texture packs, you may want to give Soartex a chance to prove itself to you.
Instead of trying to make things look overly 'realistic', Soartex instead focuses on making game textures look slick and appealing. The developer of the texture pack is Japanese, and whilst nationality would usually be about as relevant to Minecraft texture packs as a pack of melted crayons, in this case it is clear to see that the artwork used in this texture pack is very Japanese in its style. You may find yourself reminded of early video games that originated in Japan whilst using it. I especially like the light, bamboo style wooden planks.
Glowstone is also especially well done, it has an almost stained glass quality to it, which means it looks great when used as a source of lighting indoors. Windows have been replaced with wooden lattices and cobblestone has a lovely smooth stone texture that is quite reminiscent of brick. There's a lot of scope for producing very modern style creations using this texture pack, indeed my ModernCraft world, a world designed around the ModernCraft texture pack actually looked quite good in Soartex, which is a testament to the simple versatility of this pack.
Being a 64 x 64 texture pack, there are some minor inconveniences associated with using Soartex. It is necessary to use the HD texture patcher in order to use Sortex, and Sortex will replace your default texture pack, which means that you'll have a hard time using other texture packs. You can expect freezes, crashes and the infamous black screen if you do. If you've already accidentally changed texture pack and find yourself black screening, you can fix this issue by opening your .appdata file, opening the options.txt and deleting the texture pack listed. When you restart Minecraft, it will use the default textures
IMPORTANT: Note that the extra animals in the screenshot used to demonstrate Sortex are not part of Sortex, they are part of the Mo'Creatures mod, a mod I highly recommend installing if you want to add a little more diversity to your Minecraft ecosystems.