Ultima VII was my favorite of the entire series, but IV was definitely the most significant of them. It's funny, because from a pure game mechanic perspective, it was no big deal. Way simpler than modern faction systems. But in a simple game, easily expressed context - it rocked. I kinda feel it was a little bit of a missed opportunity, too… an evolutionary branch of RPGs that was never fully explored before it was dismissed and we found ourselves playing (today) endless Diablo clones.