Who's the most overrated character?

None of them. I think all of the characters are very well written and each deserve the fans they have.
TBH I just hate the term overrated as it's been super overused (and misused at that) on this site.
To be even more honest part of me isn't thrilled about this question simply because I'm 50% it's going to turn into a cesspool of Bellatrix hate.

Everyone has a different opinion on what makes for a good character. To each fan of each character; the character in mind deserves his/her fans.

Now I personally don't see the appeal of Sirius, Snape and Hermione to the degree they are loved. But I'm not going to call them overrated because I think they deserve their fans.

I think there are several over-rated characters. Snape is one of them; I agree with you there. A lot of fans adore his heroic qualities and completely overlook and excuse his nastier traits.

Another is Draco. People love him just because of the actor who portrayed him and his bad boy image.

A third is Hermione. She is a good character, and I do like her, but some people think she's completely perfect.

Bellatrix Lestrange, seriously

Hermione, Snape and the Marauders. Draco back in the old days of the fandom. Bellatrix from time to time.

Hermione and Snape.

Hermione, definitely.

Draco, for sure

Honestly, I think a lot of characters are both overrated and underrated by fans. For example, some people think Snape is perfect when he really was horrible to Harry (not to mention Ron, Hermione, Neville, etc.) and REALLY held grudges. However, some people think that Snape was evil and that his liking Harry's mom doesn't make him automatically a good person. These people don't acknowledge that Voldemort was who he was because he couldn't love, while Snape risked his life for love. The amazing thing about Rowling's characters is that THEY HAVE FLAWS. Sometimes, fans could do better to remember to forgive the flaws but don't ignore them.

Luna, everyone love her, like her,but she is not so great

Hermione and Snape

Tonks.

Severus Snape. Severus Snape. Severus Snape. Yeah, he's an interesting and complex character. He's a good character. He had good and bad in him and he did a lot of good for the light side. But some fans go overboard in making him look like a saint.
A lot of fans even unfairly demonize James Pottter just to make Snape look like a saint. Snape was not an innocent victim, even in his school days. J.K. Rowling herself said Snape was a bully during his school days. Both James and Snape made their mistakes and they both did their share of good things, yet a lot of people completely demonize James just to glorify Snape which is just not fair to me.