Feel free to refer to this list! I am an avid fan of instrumental music (including anime OSTs) and my taste is really diverse. Suggestions are also welcome!
Here are my personal favorites:
5 Centimeters per Second by Tenmon
Accel World feat. onokenAkagami no Shirayukihime by Michiru OshimaAldnoah.Zero by Hiroyuki Sawano Ao Haru Ride by Shouta HashimotoAo no Exorcist by Hiroyuki SawanoAttack on Titan by Hiroyuki SawanoBoku Dake ga Inai Machi by Yuki KajiuraCode Geass by Kotaro NakagawaDate A Live by Go SakabeDiamond no Ace by Tom-H@ ck, yamazoDurarara!! by Makoto YoshimoriFull Metal Alchemist: Brotherhood by Akira SenjuGlasslip by Akito MatsudaGuilty Crown by Hiroyuki SawanoHowl's Moving Castle by Joe HisaishiHyouka by Kouhei TanakaInu x Boku SS by Kotaro NakagawaK Project by Mikio EndoKabaneri of the Iron Fortress by Hiroyuki