Out of the 4 options, Dry Skin and Water Absorb look to be the most interesting.
Both can be used by an offensive and defensive roll, however, Dry Skin is harder to use in the Sun while Water Absorb can be used in different weather. The choice between the two, imo, depends on how you want weather to affect CaP and which weather it would prefer. Dry Skin would make CaP a valuable mon in Rain Teams (similar to Toxicroak), and is probably my favorite because it makes Poison the main STAB, while Fire is used to check 4x fire-weak pokemon such as Ferrothorn and Scizor, which is useful because Ferrothorn is a pain for Rain Teams to deal with. This would make this CaP one of the only Firemon that works in the rain (the other being Volcarona with it's Rain set). However, Sun now fucks up CaP because we lose health each turn, which is bad considering it's survivability is low as it is with weakness to Spikes and Stealth Rock. Water Absorb is much better for those who think CaP should either be unaffected by Weather or should be Sun-based, since it isn't affected by the weather. However, it makes for a great counter to Rain teams because you are immune to the Water STAB and resist the unSTAB Ice Beams most carry. I don't like Storm Drain or Drought because Water Absorb / Dry Skin could potentially heal up damage done by entry hazards.
Also, we could give it Regenerator to patch up the fact that it takes 25% from SR and just give it powerful coverage to deal with Water-types, although imo this should probably be a secondary ability.