The settings I use for most of my encodes are
--preset placebo --crf 18 --threads 8 --keyint 300 --min-keyint 24 --chroma-qp-offset -2 --merange 40 --me umh --direct spatial --subme 10 --psy-rd 1.10:0
but those are for a size/quality balance; you might want to raise the quality setting to --crf 16 for a higher quality, but I don't think going past that would be of much use.