ForeverGearScout

Type /fgs or /gearscout, or click the GearScout button on the character frame. It detects your class, level, and spec, scans everything you're wearing, and lists the best item you could have for each slot at your level. Each line shows where to get it: dungeon and boss, quest, crafted (with profession), reputation, or PvP rank. There's an "Include crafted" checkbox, an "Include PvP gear" checkbox, a spec dropdown, and a bracket picker. It covers all 9 classes with 18 spec roles across levels 30, 40, 50, 60, and PvP 60.