Clamp Gakuen Tanteidan

Both series have a group of cute guys who do things to make women happy. The main characters of each are similar (despite the fact that Tamaki is much more spontaneous than Nokoru). CLAMP School Detectives has more "mystery," and Ouran has older characters, but chances are if you enjoyed one then you'll enjoy the other. 

Both series are about somewhat shallow boys in school creating a club for detective work. Both have main protagonists who are obsessed with beauty. 

Both great CLAMP series, heartwarmig stories, elementary school characters. 

Both shows have a light-hearted plot with younger characters as the main cast. They both also have a mystery theme to them although Clamp Gakuen doesn't revolve around supernatural mystery. But the show does have a fair amount of interest plots! 

They are both great CLAMP series, and characters from CLAMP Gakuen Tanteidan appear in Tsubasa Chronicle. 

They both involve a student council and issues with a lot of girls. Plus the art styles are pretty similar. 

In CLAMP G.T., one of the main characters is really cool at making sweets, and his idea is that sweets should make people happy. In Yumeiro Pattissierre, everyone is like this! 

CLAMP Gakuen Tanteidan + Zettai Karen Children = Tantei Opera Milky Holmes Tantei Opera Milky Holmes is a lot less dry then CLAMP, but it has the abilities of ZKC... not to mention some of the humour in there. An enteresting mix. 

Both Anime involve schools for gifted children. The main characters could be said to be the most gifted in the schools. 

Detective Conan (Case Closed) is simular to CLAMP because both of the animes are about child detectives........ in detective conan the main character Edgawa, Conan is actually a teenage detective who, by certain circumstances, was reverted back into his child form. CLAMP Gakuen Tanteidan is about a group of middle school detectives who, like Edgawa, Conan from Case Closed, solve crimes/mysteries 

both plots are based on young (teenage) detectives who solve crimes/mysteries even the police force have problems with (aka they help the police solve mysteries).  

It has ultra cute characters and comedy. If your a fan of those kind of anime? then I highly recommend it to you.