Reaktor 4 is an object based synthesis environment. Think of it as a programming language. Just about any audio device can be built with it, including an arpeggigator. It is also very deep and time consuming to learn, plus it requires a great deal of background theory. If all you want is an arpeggigator, I suggest looking for a MIDI plugin that will work with whatever sequencer you're using.

well I'm basically looking for a good softsynth with an arpeggiator (I currently have reason and the only thing it lacks is an arpeggiator) and a bunch of great electronica, rave, dance, and transe sounds. I'm also hoping for something with a lot of filters and tweakability (that's why I was looking at reaktor) and plenty of effects and neat routing capabilities. any suggestions?

I'm not involved with techno music, so i'm not sure what to recommend here. Hopefully someone else will come along.