For engraving steel, glass, granite and other materials, the diamond drag engraving bit is a dependable choice due to its spring loaded floating head which keeps the pressure constant on your surface. And of course, the diamond tip can cut cleanly into most materials.
If you don't need the spring loaded pressure or the diamond cutter, The In-Groove engraving system is popular and dependable. It uses blade inserts which can be sharpened, and easily replaced when you finally wear them out. There are many different inserts available, which means you can buy an insert for different angles and materials rather than a whole new bit.
And of course you can also buy them individually from nearly any supplier, just make sure you get the right shank size for your collett, and that you have the right angle for your job.