Shimano Acera M360 Rear Derailleur

ERDM360SGSS

It indexed 7, 8 speeds and friction shifts 5, 6. 7, 8, 9, and is so cheap and all metal, and still under 300g and under $35. As long as you don’t have to index 9sp, it’s the best derailer value in Shimano’s line and the world.

We sometimes talk about how great Shimano's inexpensive derailers are, and this one exemplifies that as much as any. Holy Toledo. If we knew Shimano was going to discontinue it, we'd buy 1,000 of them, seriously. I / Grant have one on my Appaloosa, and as much as I like and prefer RapidRise models (and have access to them more than you do), I would never remove this to put on anything else.

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.