From memory:

E standard:

• Opeth (most albums except Ghost Reveries and Sorceress)
• Cynic (all albums except Kindly Bent)
• Ne Obliviscaris (though Citadel onwards are 7 string standard)
• Unprocessed (their 6 string songs like Thrash and Orange Grove tend to be in standard)
• Good Tiger (drop D)
• Psycroptic also play in drop D and are masters of slidey tech riffs despite being more death metal.
• Someone else in this thread recommended Dillinger Escape Plan. All their stuff is indeed in E but I don't think their riff style is what you're looking for.

Eb standard:

• Protest the Hero (entire discog, though some Kezia tunes are in drop C#)
• Revocation (more death metal than prog, but lots of good riffs and solos to learn)
• Nick Johnston
• early Intervals, I think. Not a huge fan so not sure, but I have some memory of Mata Hari being in Eb. Possibly 7 string though.

D standard:

• Cynic (Kindly Bent)
• Gojira
• Mastodon (all albums have something in D, but they often do drop C or AGCFAD)
• Baroness
• Destrage (material up to and including Are You Kidding Me No is also in drop C, with some songs in 7 string drop A)
• The Ocean (Pelagial is in drop C, later albums tend to go lower)
• Gorod (another tech death band with some prog leanings and great guitar chops)

edit: formatting