While some idol groups (particularly the newer ones) debuted with a strong image that they've kept to this day, there are other groups that have experimented with different sounds and images before they developed into the top artists they are today.  So let's take a look at the 10 idol groups that have matured the most since their debut!  

----

U-Kiss

U-Kiss has become one of the most popular K-Pop groups internationally.  When they first debuted, they had hits such as "Man Man Ha Ni."  Although they have remained true to the quintessential K-Pop dance track sounds (with great ballads mixed in to each album, my favorite being "0330," as well), they have visibly grown as artists.  Their vocal abilities as well as their dancing are on point and harmonize beautifully. 

f(x)

Ever since the beginning, the ladies of f(x) have always had an extra edgy feel to them that separates them from other girl groups.  Although they have stayed true to their original electro sound, these girls have really matured from their "Chu" days.  I think "Pinocchio" was their turnaround track that really established the definitive f(x) sound.  f(x) has a distinct sound and fashion that not only is unique and distinguishable, but something only f(x) can pull off the way they do. 

SS501

Although this choice may seem a bit random (probably because it is), SS501 comes to mind when I think of groups who have matured and developed their sound and image from when they first debuted.  When they first came out with tracks such as "Snow Prince," they reminded me of vintage Super Junior.  However, they evolved into tracks such as "U R Man" and finally their latest track that they released together, "Love Ya."  I'm looking forward to them releasing another album some day as a full group!

Girl's Day

Girl's Day has changed a lot since their original debut as a 5-member girl group.  Aside from the obvious member change up, their musicality has changed as well.  One of their first tracks, "Nothing Lasts Forever," was a song about female empowerment—they then reverted to aegyo-filled songs for their next couple of promotions before growing into sexy tracks that become mega-hits today, such as "Expectation." 

Big Bang

Big Bang debuted with a 90's vibe hip-hop image and sound.  However, they have since traded in their oversized baggy clothing for a more refined high-fashion style. They have each found their own individual trademark fashion sense.  And it's not only their style that has developed—their music has as well.  We're now treated to an artistic mix of hip-hop and electronic music that Big Bang has made their own. 

After School

After School is another group that has gone through many member changes—their "graduations" keep their group young, fresh, and always relevant to the music scene.  Going back (not all the way back, but pretty back) to their "Bang!" days, After School always had catchy songs and great choreography (and of course execution of the choreography).  Although much of this remains the same, After School is even more daring with their sound and visuals with their latest singles, such as,"First Love." 

Shinhwa

When a group has been around as long as Shinhwa, it's natural that we see many differences between their debut image and current image.  Shinhwa is a first generation idol group (they debuted in 1998), but still remain 100% relevant and continue releasing chart-topping music.  They went from blonde, crazy hair that was popular in the 90's to a much more mature and manly chic image, and their music has become cleaner and crisper over the years as well. 

SISTAR

When SISTAR first debuted with "Push Push," I thought they were going to be a bubbly girl group based on cute concepts.  However, since then, they have established themselves as one of the top female groups in K-pop, and dominated charts with their strong lyrics and sexy concepts--although they do jump back to bubbly songs (e.g. "Loving U") from time to time which is always a treat. 

Super Junior

Super Junior always caught people's attention from the start—with all those members on one stage, it's hard not to notice them.  Considering they've been around the K-pop scene since 2005, it's no surprise that the group has matured (especially when all of them suddenly decided to hit the gym and come back with abs).  Musically speaking, they have been consistent with their sound since "Sorry, Sorry," and really followed the, "if it ain't broke, don't fix it" saying with their follow-up promotions. 

Girl's Generation

Girls' Generation released their first single "Into the New World" when they were just teenagers.  They had a youthful image that was completely natural—they didn't need overly frilly skirts or hair bows to show their innocence.  Within a span of 7 years, the girls, from their fashion to their music, have matured into Korea's poster girl group. 

That rounds out the top 10.  Did your bias make the list?  Who did I leave out? Comment below!