By far Blood Omen is the best out of the series and had the darkest tone, a gothic feel to it. Then Soul Reaver 1. SR2 was a fun game to play. BO2 was fun in it's own right, but the gameplay was definitely a step back. I agree w/you on Defiance. It gets a bit repetitive and stale in comparison. But I also loved being overpowered w/Kain's abilities and toying around w/the Serafan.
Every few years I'll go back and play the series in different order. That's what's so great about it, is the timelines interwine w/eachother, that it doesnt matter which game you start out with. You can start out w/Defiance and begin w/the rise of the Hylden in BO2. Or start w/BO2 and proceed into the SR 1 & 2 series. Or simply go straight through the order in the aftermath of Defiance w/BO1 & BO2 because of the time alteration.