Likely gives the most mod coverage - Faithful 32x modded; Looks like vanilla but in 32x textures and covers every mod under the sun if you use the latest releases. Alternatives:

Soartex Fanver or Invictus Modded Universal - Cover a broad range of mods but individual modpack editions are also a thing from their site. Super clean and modern-looking, but very demanding on PC resources.

John Smith: Technician's Remix - Medieval styled, so it's good if you want a rougher-looking pack that looks nothing like vanilla. Has a lot of coverage and isn't as tough on PC resources as Soartex but it's still quite large.

Sphax Pure BDcraft - The pack itself is vanilla-only but mod support has its own subforum on their site so you can pick up individual patches for specific mods or packs. Be warned; using the higher-res versions will utterly destroy all but the best PCs, so don't overdo it. :p

Unity - Sort-of vanilla styled with a darker, grittier look. Doesn't necessarily support every mod you come across but it aims to make everything blend together well, so when it doesn't cover something it's usually not that noticable. This is one of the few packs to provide nice-looking textures for RotaryCraft.

Another one I personally like but doesn't have quite as much coverage is Lithos. The "core" pack by itself is similar to faithful but has a lot more detail and richness. The Luminous addon makes it look even brighter, and the stained glass textures are to die for. :3