Always a fun experiment, tho I hope I'm never in a position to rely on it for survival!

Note: as @SurvivalGal points out, go for nice green leaves. To flesh this out: First, healthy leaves are going to be doing more work, and producing more water. But, also, plants transpire at different rates - large soft leaves are likely to be grown by plants that throw their water around and are less efficient in retaining it, and thus provide you with more. Waxy, tough, arid adapted leaves are probably going to be releasing less water, and conifers are much less water-giving than many other trees.
Individual plants can also vary: some plants have both "shade" and "sun" leaves. Typically sun leaves will be at the top/outside of the plant, smaller and tougher, and will transpire less, giving you less water. Shade leaves, because they are sheltered, are used by the plant to do the grunt work and will be giving up more water under the same heat/humidity conditions (but are overall more efficient for the plant). Also as mentioned above, being exposed to sun is important as plants use transpiration to cool their leaves and will up the rate if they are hot, so it can be extra helpful to make sure that you clear any obstructions from the leaves you bag, including exposing shade leaves if you have them. The rate at which leaves let off water can be measured and can drop to almost nothing even when the sun goes behind a cloud, so don't underestimate exposure to sunlight in increasing your yield!