Good old mattress stitch is always my first choice for a seam that needs to look good. The instructions for it in The Knitter’s Handbook pair it with what they call “fake grafting” for the horizontal seams, but the zigzag motion is similar so I’m calling it all mattress stitch here.
It’s worked with the right side facing you, and it creates an attractive, low-bulk seam. The vertical stitches almost disappear into the fold of the seam.
I can’t think of any projects where this seaming technique would be a bad choice. The only downside to it is that you do have to guess the length of yarn you’ll need. It can be somewhat challenging stitch to unravel so it’s not well suited for basting, but it makes a nice, sturdy final seam.