For me

Soul Blade: Awesome
Soul Calibur: Eh, alright, missing some of the single player magic of Soul Blade
Soul Calibur 2: Great line-up, I had it on the Gamecube, so LINK FTW! Also fantastic adventure mode
Soul Calibur 3: The pinnacle, very indepth character creation, fantastic single player challenges
Soul Calibur 4: Massive let down, just felt like an advertisement for Force Unleashed
Soul Calibur 5: Great online modes, but at the expense of pretty much everything else

If they continue this series I really hope they put as much effort into offline as they for their online offerings. The adventure modes in 2 and 3 were highlights of the franchise for me.