The Shimano Altus (M310) was an inexpensive, trusty, reliable, honest, steel derailleur which I rather liked. It changed crisply and its massive pulley wheels gave a usefully huge capacity for such a compact unit.
I have included it on this site as it was the natural heir to Shimano's Nexave flight of fancy. It had the Nexave silver painted finish (and those massive pulley wheels), but eschewed the sci-fi styling, Rapid-Rise low-normal logic and crazy cable routing. Most importantly it was intensely affordable - as any hard-working urban workhorse derailleur should be. From 2010, Shimano acknowledged this by showing the Shimano Altus (M310) as part of the Nexave T300 groupset.
In that terrifying Shimano way, in which every defeat is turned into a victory, the horribly slow-selling Nexave series gave rise to the Shimano Altus (M310), which sold in its billions.