I coudn't make up my mind, so I got all four:
1. An A5-ish (American) board that straps to my knee and is perfect for writing down stuff while airborne. It has a fiercely strong clip that will hold whatever you grip with it. I use it all the time;
2. An A4 (Transair) one with a plastic cover (keeps plogs dry while sprinting to the aircraft in the rain). It holds the detailed A4 plog and has pockets for various bits;
3. The American "IFR flight file", a roughly A5 size spiral-bound contraption with loads of placcy pockets for Departure, Destination, Alternate, and a few more sets of IFR (or VFR arrival) plates. Brilliant device, because you can open it right back and clip it to...
4. A yoke clip that will hold approach plates and display them clearly while both hands are occupied trying to land the beast. Also has a mount for the trusty stopwatch.
Wouldn't fly without all 4, unless only on a local jolly.