Having put several scratches in the front of my tank using metal edged scrapers--never using 'em again. Ever. Especially the handled ones as it's too easy to have 'em twist or get at an angle that puts an edge wrong against the glass.
Any stiff bit of plastic can work well--my favorite for tough jobs is actually a clay carving tool with multiple edges: curved, flat and a 90 degree edger. For lighter weight scrubbing, tight areas and my pico tanks I use a long bamboo skewer with one end wrapped with floss; the floss is just abrasive enough to scrub off algae without damaging the glass let's me get right in against the substrate and behind hard scape pieces.