There has always been a lot of criticisms against having idols becoming actors, especially in the recent years where Japan, Taiwan and even more so Korea saw an influx of idol actors. Many criticisms stem from that majority of these idol actors lack acting skills to properly portray the emotions that's required of them, and I'd say they are rightly justified in claiming so. Compared to full-time actors/actresses at spend their whole life training for the acting career and improving themselves in that area but lead roles are given to idols that lack the skills needed and get the roles simply because well, their idols. In doing so, it's not only unfair to the actors/actresses and also do not do justice to the role that could have been better portrayed by a professional.
However that's not to say that there are absolutely no idols that are good at acting. If any, the ones highlighted below really deserve to be commended for their acting skills.
1. 二宮和也 Ninomiya Kazunari: He's simply the BEST idol actor I've watched. From Hollywood movie "Letters from Iwojima" to protagonist in "Tengoku de Kimi ni Aetara" and "Ooku" to semi-antagonist in "Ryuusei no Kizuna" and "GANTZ", a poor kid in "Yamada Tarou Monogatari" to a kid suffering from Ewing's sarcoma (similar to cancer) in "Sukoshi wa, Ongaeshi ga Dekitakana", an autistic kid in "Marathon" to a cerebral palsy young adult in "DOOR TO DOOR", he's acted in them all and superbly as well. If you try watching his movies/dramas in chronological order it's not hard to notice how much he has progressed as an actor over the years. It's hard to explain how good he is at acting, but someone of his caliber is hard to come by among even among actors, a true talent. I am just waiting for the day that he ends up acting as a killer.
2. 大野智 Ohno Satoshi: He doesn't have as long a list of hit works as Nino does, but those roles he undertakes he does well in them, and he's quite the versatile actor. From the lawyer that's also a killer in "Maou", who could have expected him to be the same as the cute and bubbly character in "Uta no Onii-san" or the childish but adorable "Kaibutsu-kun". I was especially impressed by Ohno's role in Maou actually. He may not act a lot but he'll be quite the force to reckon with when more works come his way, not that he's not already one.
3. 生田斗真 Ikuta Toma: I'm not sure if he can be considered an idol but it doesn't make him any less an outstanding actor. The only reason why he's not ranked higher than Ohno or Nino is not because he's not better, only because I haven't watched enough of his dramas to rank him that high up. I've only watched "Voice", "Maou" and of course his hit work "Hanazakari no Kimitachi e", and I have to say he pulls off both the serious and comical/jokester roles really well. No matter how many times people remake HanaKimi, I still don't think anyone can pull off Nakatsu as well as Ikuta did. Well Ikuta was initially trained to be an idol, but I guess himself and his jimusho figured out in the end that his gift lies in that or acting, not performing.
4. 錦戸亮 Nishikido Ryo: Everyone would remember Ryo from "Ichi Rittoru no Namida", the most famous tear-jerker drama. Those that are brave enough would also remember him from "Last Friends", a guy guilty of DV (Domestic Violence). Although I hear many say watching Last Friends would ruin their image of Ryo forever and be stuck as one of DV, but that's also proof of the success of Ryo's acting. Well there's also the cancer patient in "Niini no Koto wo Wasurenaide" and not forgetting the more brawns than brain comic relief acting as Nino's brother from "Ryuusei no Kizuna". Ryo is also a superb actor, except it's quite the pity that it appears that his looks does limit quite a bit of his roles in dramas.
Do note that the above are strictly only idols. If I were to even consider the professional actors the list would be endless, what with Eita, Daito Shunsuke, Oguri Shun, Narimiya Hiroki, Yamamoto Yusuke, Miura Haruma just to name a few. But of course also remember, the ones listed above although carrying the idol status, are as good as actor as any professional actor.
Sure many idols, regardless of country's industry, have done considerably well in acting, but as far as it is for me they are decent or good as an IDOL-ACTOR but they still have ways to go to be worthy of the title Actor. So before anyone starts protesting about their idol-actors being worthy of carrying the status of an actor, unless they can match up to the ones I've listed above, I'm sorry but they're not there yet.
I just felt like adding this, but these actors are worth looking out for. They have the potential to become outstanding actors in future. Yoo Seungho, Kim Soohyun. (Note that Song Joongki is not included, because I think he is already past the potential, in other words he's a wonderful actor.)