Oh, the Regis :D I quite liked their concept, it was so very different to the past legendary trios we'd had, even without considering the manner in which you get access to their caves. I remember there's a part where you need to use Dig, and typically when I worked from the Braille chart, I mistranslated it to 'Die', which spooked me for a second :P
Of the three, I'd probably like Registeel the most. I like his design much more than the other two, and in general he'd have a type advantage over them. Aside from that he was the first legendary Steel-type, which was pretty cool at the time. However, I've never actively used/trained any of the Regis, to be honest.