From a design standpoint, Soul Calibur is the perfect interpretation of a medieval Street Fighter. Think about it, the game follows (or at least used to follow) the principles of what makes SF designs good, they used the same exact formula for both generating the cast as a whole and designing each individual character. It also has the same basic concept: very different people from different cultures fighting each other.
From a gameplay standpoint, Soul Calibur is the most fluid and deep 3D fighter you can get without making the game extremely difficult to play, like Virtua Fighter. The use of weapons makes movement and positioning in all three X, Y and Z axes key to success in a fight, unlike in other games where crouching has no big impact against most characters, and moving three feet away from the enemy will keep you safe until you figure out what to do. You're also given a lot of freedom to play your own way, with amazing movement and defensive systems unless you're playing SCV. However, button mashers and amateurs can also do well against the CPU for the most part, which is a plus for casuals.
You also have to remember how extremely polished the games in this series are. The graphics are always the best in the genre whenever a new game comes out, both characters and stages have several layers of detail in the modeling and texturing. Animation is flawless, from attacks, to faces, to the bunnies behind the trees in Viola's stage, everything moves with a level of beauty and grace above anything else in the market. And let's not forget how EPIC the music is. And although this has been fading over time for some odd reason, the amount of single player content is (or was) simply above all else. You had arcade, mission mode, survival, team battle, character creation and several others. I mean, some of those missions were very cleverly made, and could both entertain and challenge you in very unique ways, with my personal favorites being the coliseum from SE, the lizardman invasion from SC, and the one where instead of lifebars, you and the enemy were given a certain amount of hits of resistance. These were simply genius ways of adding fun to the game while keeping the gameplay completely unchanged, and that was a feat back in 1998 fg's.
Ever since the very beginning, the SC series has been the most fluid, beautiful and innovative fighter in the market. It's pure awesomeness.