Recently, Jim Parsons expressed interest in playing The Riddler, and reportedly Jim Carrey desires to reprise the role. Yet, for the longest time I've thought and still think Neil Patrick Harris would make a great Riddler.
Now, there's the question of which Riddler would he fit best? With so many versions from Frank Gorshin's campy Riddler, Carrey's wild rendition and his recent portrayal by Corey Michael Smith in Gotham. I'd personally go with his characterization in the Batman Arkham video game series. The Riddler's personality in these games resonates a buffet plate full of everything. He's manipulative, insecure, comedic, sadistic, egotistical and psychotic. In the games, he leaves clues for Batman to find (whether it be for the vigilante to locate him or save victims from Riddler's death traps). The Riddler's strong character here is attributed to the stupendous writing and his actor Wally Wingert.
With NPH's comedic sensibilities famously shown in How I Met Your Mother and the Harold & Kumar series. Taking on the Riddler's humorous side doesn't seem like it'd be a stretch for him. Especially since he's handled black and tongue in cheek comedy exceptionally well. Same goes with playing a cocky and entitled person such as Barney Stinson. Just like the Riddler, Stinson's sometimes blinded by his own cunning and pretentious attitude that he can end up in trouble. Whether it be with Batman or the ladies.
And let's not forget his short, but fantastic stint on American Horror Story: Freak Show. Harris is a flamboyant magician/puppeteer/salesman named Chester Creb who has an unnatural relationship with his puppet Marjorie. Creb believes Marjorie's alive and has killed his own wife, her lover, Maggie and almost the twins (Bette and Dott) because she ordered him to. Creb also would occasionally hallucinate and see Marjorie as a human than a puppet.
So, mental instability, check! Although, I'm sure a different version or his own interpretation would be just as intriguing, Arkham Riddler is one I'd love to see him tackle.