You need to chose what works for you, and some trial and error may be pertinent here. Compared to the cost of flying knee boards are pretty cheap to cycle through. There are a few questions to answer that may be impacted by your training and the type of flying you are doing:
Am I an iPad Pilot? These days many people use iPads in the cockpit to replace paper charts. There are lots of options out there for iPad knee boards and with the scratch pad ability in most flying apps you can replace paper all together in the cockpit. If this is the way you fly you will want something sturdy that can hold the iPad in place. You may also want to consider a yoke mount.
Am I a paper pilot? If you like sectionals and a note pad there are various clip style ones out there. You can build one or buy it but I like the ones from the various major aviation suppliers as they tend to have some useful aviation numbers like weather minimums and those pesky tower light signals printed on them for reference. The military style ones tend to have a nice pen holder if thats what your after.
Do you want a side pocket? Be it clip or iPad style most knee boards I have seen are either single plate or dual in that they have a side pocket that hangs over (there are even some trifold ones out there I belive). The side is good for holding gum, extra headset batteries, a pen/stylus or an iPad cable (at least thats whats in mine).