Day One is the best option for journalling from multiple Apple devices (including Macs) because new entries are synchronised automatically using iCloud or Dropbox. It doesn't have many personalisation options; the emphasis is on providing a clutter-free interface that makes it easy to add new entries and look through existing ones.
My Wonderful Days is the cheaper alternative to Day One, with apps for the iPhone, iPad and Mac computers synchronising via iCloud. The difference with My Wonderful Days is that it lets you make only one entry a day. It has a fun, quirky interface and offers a few unique features such as note statistics and a mood slider.
The brilliance behind Momento is that you don't have to make an effort to keep it up to date. Momento pulls data from the services you already use, such as Facebook, Twitter, Instagram and Foursquare, and uses it to create a diary for you. You can also add entries manually using a combination of plain text, photos and tags.
My Daily Journal offers a more customised experience than the other apps. You can change the background of each page using one of the paper styles or your own photo, and you can mix it up further with different fonts for each entry. However, you can't add photos as attachments, nor is there an automatic sync option.
Diaro 3 covers all the bases for Android users. You can add multiple entries a day, attach unlimited photos and organise entries by categories and tags. You can also customise the background and font that's used for each category. If you want to sync between Android devices and get rid of the ads, you'll need to upgrade to the Pro version for $3.99.