Shimano Acera M360 Rear Derailleur
It friction shifts five through nine speeds, probably even ten, but five thru nine for sure, and is so cheap and all metal, and still under 300g and under $30. As long as you don’t have to index 9sp, it’s the best all-metal derailer value in Shimano’s line and the world. I can’t see Shimano continuing to make this. They have a new 9sp indexing model, but it doesn’t look as nice. It looks all SRAM-y. This is the one to get.
The only other two to consider:
Shimano Altus M310
-7/8 index or five through nine friction;
-Pros: Bigger pulleys allow shorter cage, nice compact look.
-Cons: More plastic.
Shimano Deore M591
-9sp index or five thru nine friction
-Pros: If you index 9, that’s a pro.
-Cons: Smaller (normal) pulleys, so it has a longer cage. Visual con only, not functional, and not a con to everybody.