Among WMD, by far, the most destructive is nuclear. In South Asia, India and Pakistan continue pursuing nuclear weapon and missile programs in the context of a tense regional dynamic whose dangers are routinely obvious in the headlines. Iran is pursuing a dedicated nuclear weapons development effort. Desert Storm and the subsequent UN sanctions regime interrupted Iraq's advanced nuclear program, but that country maintains a human infrastructure that could build nuclear weapons if unconstrained. North Korea, also, appears to maintain nuclear aspirations.
Biological weapons are relatively cheap to make and wantonly destructive.
Tiny amounts can wreak unprecedented destruction. Roughly a dozen countries have active programs. Chemical weapons, by contrast, pose significant but smaller-scale threats. More ominously, chemical and biological weapons are attractive to terrorists. Iran has the capability to reach most of the Middle East and is working on longer-range missiles.