It's not really possible to say what's "best" because the criteria is super subjective.
I personally like the Spline family, and I default to Spline36 for both upscaling and downscaling in ffmpeg. Most people can't tell the difference between Spline36 and Lanczos3. If you want more sharpness, go for Spline64, for less sharpness, try Spline16.
Edit: As far as I'm aware though OpenCV doesn't have Spline as an option for resizing.
I personally like the Spline family, and I default to Spline36 for both upscaling and downscaling in ffmpeg. Most people can't tell the difference between Spline36 and Lanczos3. If you want more sharpness, go for Spline64, for less sharpness, try Spline16.
Edit: As far as I'm aware though OpenCV doesn't have Spline as an option for resizing.