I tend to play a lot of singed myself. I just seem to do better with him than others. But IMO you can build Singed for either tank or AP if you want to. I lean more toward a tank build mainly because people don't focus on me much until the first couple people on their team get flung into the group of nukers behind me. Usually I end up getting boots/SC/force of nature (and a banshee's veil if I am up against a heavy magic team). By the time they figure I might be a problem they are hard pressed to damage me a whole lot. But then again there are those off games I have every now and then. :)