Zyprexa (generic name = olanzapine) is the worst medication for weight gain, beyond any doubt.

Wikipedia:

Of all the atypical antipsychotics, olanzapine is one of the most likely to induce weight gain based on various measures.[26][27][28][29] The effect is dose dependent in humans[30] and animal models of olanzapine-induced metabolic side-effects.[31] Olanzapine may directly affect adipocyte function, promoting fat deposition.[32] There are some case reports of olanzapine-induced diabetic ketoacidosis.[33] Olanzapine may decrease insulin sensitivity,[34][35] though one 3-week study seems to refute this.[36] It may also increase triglyceride levels.[27]
Recent studies have established that:
olanzapine and clozapine disturb the metabolism by making the body take preferentially its energy from fat (instead of privileging carbohydrates). Thus, levels of carbohydrates remaining high, the body would develop insulin resistance (reduction of insulin sensitivity).[37]
olanzapine promotes fat accumulation : due to disturbances in fat metabolism, rodents become fatter (but don't have their weight increasing at first). Being fatter, they do less exercise, burning less fat and gaining weight.[38]
olanzapine may cause body weight gain and hyperphagia by altering appetite signaling in the brain and periphery[39]
olanzapine may induce hyperglycaemia leading to diabetes side-effects by altering insulin secretion from the pancreatic beta cell through blockade of the muscarinic M3 receptor[40]