I recommend the Bride Guardis III.. Great seat, very strong and quite comfy for everyday use. A little awkward getting in and out but you get used to it. Can't really use a standard seat belt with it though... Not really cheap but you want a good quality seat don't you?
http://www.more-japan.com/Bride-GARDIS-III-p-1-c-407.html
http://www.rhdjapan.com/jdm-low/BRIDE-GARD...-Rail-Set-15251
I've also used the Bride Zeta III full bucket seat, also a nice secure seat. Some people complain about the narrow lower back and seat base, so if you have a fat ass might be worth trying it first. They come in a wider base version too.