It really depends on your budget and the people you're giving it to. I'm making my personal afghan out of wool (not superwash), but I know that I will take care of it properly. For someone who won't, or who I'm not sure of, I'd go with either a superwash wool or a wool blend. Wool blankets can be used year-round where acrylics are mostly relegated to winter-time, because acrylic doesn't breath. I'm making mine out of wool because I get easily overheated in the winter being bundled up, and wool is supposed to help that.
But, the flip side of that is that wool is expensive. Although I got enough wool to make 2 or 3 afghans off ebay for $100, that is still more than you would pay for acrylic.
For a nice fluffy afghan, double-stranding Lion Brand Homespun makes a lovely soft blanket, but it does have a tendency to pill, and you won't get any stitch definition out of it.
I'd say pick what pattern you want to go with, and then decide on yarn, since the pattern will dictate if you need a smooth yarn or multiple colors, etc. There's also any number of afghan KALs if you find a pattern you'd like to do.