Apparently there are so many ways to write {かっこいい}.
Hiragana/katakana only:
EDICT:
Other possible variants:
In general, which is the most commonly used variant? Are there also specific places where one variant is preferred over the others (including the common one)?
Some native feelings about the different spellings:
かっこいい is only the result on the one of my Japanese-English dictionary and has most hit according on Google search results. So I would like to assume かっこいい most commonly used one.
And there is now decent one char (unicode) variant for かっこいい, which is "△"
"本田△" → 本田さんかっけい（三角形） → 本田さんカッコイイ
Google fight! (I'm lazy.)
I think anything below a million here is pretty much not used.
かっこいい is the one to go for. The ones with katakana add emphasis and can be used as you like. Edict often has strange results and isn't to be trusted completely.
I think if I was going to use the 良い kanji, I'd probably go for 格好良い (which is a little different, the 格好 being かっこう). I would have thought the meaning for 格好いい was more "nice appearance" than "cool".
In general, people will use the hiragana version かっこいい. If you talk with friends and use 格好いい, they'll most likely be thrown off for a bit.
かわいい has a similar usage, with the reason being that just using hiragana makes it feel "cuter" since children without kanji experience are more likely to use it.
It's important to note that you might also find the net-slang version littered about message boards and other popular sites かっけえええええ. Because it's slang, you shouldn't use it around people you don't know though!
In my dictionary I only get these variants:
かっこいい and カッコいい seem to be more common in dialog, while 格好いい is more common in prose. If you were quoting someone or writing an informal email, tweet, etc., you would probably use かっこいい or カッコいい. But if you were writing from a third-person perspective, you would probably use 格好いい.
So you have 2 ways to write it: かっこいい & かっこういい The only variants I encountered in mangas until now were: カッコイイ and 格好いい (the latter is long, as you can also see, when you write かっこう and presse enter, you'll get the kanji's, but not if you write only かっこ)
I believe the most popular form is か っ こ い い, at least among the most popular search terms cited here. However, you can test other variations and update the graph in Google Trends. Thanks for listening.
