skellies, id say
the thing with tomb kings is you need to really master the 'magic' combos of the tomb kings/lich priests. once youve figured out what works best you should be ok
though seriously, the core of tomb kings are skeletons, and dont forget chariots.
regarding more special things, screaming skull catapults and the casket of souls are both very good (though being both rare, difficult to choose between). ushabtis are a general monster and great models. and swarms are great too