私は〇〇（グループ）のファンですって英語でなんて言うの？

Usually, when you say that you are someone's biggest fan, you should use the phrase "one of," as in "one of your biggest fans."
What you are saying is "you have many fans, but I am one of the biggest."

You can also say something like, "you guys are my favorite group," or if it's not a group, just "you are my favorite musician." If you want to write more, you can tell them something like, "I love listening to your music!"

Actually, you can put it all together to say something like this:
- Hi, I am one of your biggest fans! You guys are my favorite group and I love listening to your music!

"I'm a huge fan." expresses to someone or a group that you enjoy a band/ singer/ writer that you enjoy.

Follower refers directly to social media and your love of the group.

Any of these can be used.

To say that you are obsessed with something implies that you are constantly talking about or thinking about something. It expresses a deep love for something. To say you are obsessed with someone's music conveys that you prefer it to others. You may also say you are an admirer of someone's music. It has the same effect.

1. "I just love cooking." "I just love going for long walks by the sea." These are expressions telling people what you really like to do - or in this case, the artist you really like.
2. "Justin Bieber kicks ass!" If somethings kick ass, its 'Really cool' or 'Great!' (slang)
3. "This tells people how you feel when you listen to their music."

>I am your biggest fan.
>I love you guys! I am you biggest fan.
*Fan=Supporter

Other examples
a. I am your biggest follower.
b.I am one of you top supporters.

When you want to say that you're a boy-band's fan on their Facebook page, then you can say something like:
-I'm your biggest fan.
-I'm a fan of your work.
-You are one of my favorite bands in the world.

To say that you are a 'huge' fan does not mean that you are physically big in size, but that you have a huge amount of love and respect for the band or person you are a fan of.