I bought my first moleskin diary quite a few years ago and since then I've dabbled in different sizes, colours and other brands. I'm super fickle when it comes to diaries if I buy 18 month editions I've usually changed it up in 9 and if I buy a year I've switched to a mid year diary within 6 months. If I'm honest I'm yet to find the diary that does everything - in my head I know exactly how I would want my diary to be but I can't find someone who makes it. Maybe I'll just make it myself some time ... however in 2013 I made my way through just one diary for the entire year and that was the black large moleskin.
Moleskin is the brand that comes the closest to what I do dream of for many reasons. I love the matching elastic that holds it together, the pouch at the back for storing vouchers and bills. The week to a page and simple layout. The crazy amount of extra info at the front including stuff on time zones, currencies and measurements. A 6 month page to view at the front for jotting down birthdays and big events that you can quickly glance over. Overall it has everything you would want in a diary. I dabbled in a pocket edition for the first few months of this year - it didn't go well. If you're a woman with lots of personal info you need to jot down stick with the large edition (a little longer than A5 but just as wide.) If you use one diary for work and personal life I'd recommend the extra large edition. They come in a variety of colours and styles here.
Moleskin also produce a wedding journal that looks perfect if you're planning your big day. A baby journal if you're expecting. I always pick mine up from John Lewis and they range from about £14 - £18 and are well worth the pennies.