If you knit, you may already know about the SSK decrease, which is in my opinion the best looking left-leaning decrease. It involves slipping two stitches knitwise, one at a time, and then knitting these slipped stitches through the back loops.
You can also work this decrease without taking the stitches off the first needle. It especially works well with wooden or bamboo needles, but metal works, too.
Here's how to do it (captions are below the photos they reference):
Insert the needle knitwise into the first stitch, but do not take it off.