mono ice. it does absolutely nothing. nothing at all. no strengths, a subpar stab, absolutely no worthwhile resistances and a ton of hindering weaknesses.
AT LEAST rock/fire gets a sp.def boost from sand and ice/flying can be neutral to fight and immune to ground. those are still the 2 runner ups for second worst though.
I agree than mono Ice has awful weaknesses to common moves and no resistances (its only resistant to its own type), but to say it has no strengths seems a bit prejudiced. They get precious SE damage on Dragons (which is even better when you factor that quite a few Dragons have secondary Flying type), and as an attacking type they can do quite well. (A lot of Pokemon that can learn Ice type moves very often carry them as coverage, particularly Water types who almost all get Ice Beam)