Android is the best mobile device to type in Japanese.

Before iOS and Android

Dream on. On feature phones, I could not type in Japanese. Actually there was one way... The Korean character set contains some Kanas as special characters, so I could select one by one and insert them. What a pain in the... lower back.

Then, on Symbian... You have to separately buy a third-party Japanese IME, and it was not even good. And to switch the input mode... A real pain in the lower back.

Until 2010, iOS was the King.

Until 2011, typing in Japanese had been agonizing on Android devices. OpenWnn was unusable because it constantly crashed on all of my Android devices, Simeji was a better one, but it got a strange key layout and the candidates were puny and weak. I mean, it could show candidates for a single word,  but it could not show candidates for a sentence. To get that, you have to press the 'social-conversion (I forgot the exact term)' which probably sent the text to a server then got the candidates from it.

There were other multi-language IMEs like Go Keyboard. While it was very convenient for switching between languages, it did not provide good Japanese features. Candidates were also weak and they strangely placed "?" in a wrong position in T9 key mode (I reported the team, but they did not fixed it for a while.)

iOS was enjoying the decent Apple-made Japanese IME. It showed good candidates, it could show candidates for sentences. It even seem to understand the word forms, relations to other words in the same sentence (showing different Kanji for the same Kana, according to other words near-by).

Now, Android is the King. The Google Japanese IME.

I use Google Japanese IME on my MacBook and Windows, so I know that Google Japanese IME is good. The Android version is also great in its conversion performance. I love the T9 mode (like the picture below) and it just works almost perfectly.

It suggests previously typed words and phrases intelligently, so that I can almost type Japanese on my Galaxy Nexus as I do on my PC. The candidates are ample and it can show candidates for a sentence without Internet connection. Now, I have iPad running iOS 5 something, but definitely, Google's one is showing better candidates. I think Google's candidates are more up-to-date. It can suggest famous people's names, places, company names, buzz words... It really does help me.

Yeah, nothing is perfect, but...

There are some downsides, though. First, the English mode sucks. Since I also often need to type in English, I switch to the English mode (because switching the IME itself on Android is cumbersome), but the English mode shows no English word candidates or corrections! It just shows the Japanese words for that pronunciation. What a stupid behavior. At least give me some choice to set the candidate to English.

Secondly, I think it would be much much better if I can log on the IME with my Google account, so that personalised candidates are shared between all my Google Japanese IME instances (spanning 5 devices!). Or at least give me a decent way to export and import the personalised data.

I really hate the Japanese font on WP 7.5, and the IME is not good either.I stolen this picture from a Google Image search, sorry. I did not want to boot up my WP.

But these are just minor inconveniences. On iOS/Windows Phone, you cannot use third-party keyboards. I own a Windows Phone 7.5, and its Japanese keyboard was not good. So I dare say, Android is currently the best mobile device to type in Japanese.