You'll eventually want to do all of the above, so you won't make a really regrettable decision. However I would do it in the following order.
Scarlet Witch. She has the greatest utility in all of the WBUs, including as the best striker. Give her a mind damage, damage proc obelisk.
Deadpool. CTPE to maximize your investment in him.
Cable. Since you don't care about ABX above 100k (wise choice), the CTPE is a luxury.