You don't need a keyboard for your iPad. But, if you're planning on an iPad being a laptop-replacing device, having one can come in very handy if you're planning on getting writing done. I've played with a lot of keyboard accessories on the iPad, but these are my favorites.
Before the iPad Air, most cases would fit the iPad 2, 3 and 4th-gen models equally. That's no longer the case. So, having used several new solutions and written on each, I've broken my recommendations down into category of iPad: Air, 2/3/4th-gen, and iPad Mini.
You could also, of course, go with a keyboard that's not a case at all. The advantage: it works across models. I have a few older recommendations at the bottom, but I'm sure there are others, too.
Keep in mind that none of these support trackpads, because iOS still doesn't allow for it. Sad, but true.