User blog comment:Ultimate Mega Gamer/UMG's Weekly Thumbnail Contest 2/@comment-26902116-20171120062747/@comment-30898563-20171121001920

This one's not really popular: Ash Ketchum VS Lucy Hearfilia

The rest probably are:

Ryuko VS Akame

Ruby Rose VS Maka Albarn

Genos VS Cyborg

Saitama VS Goku

Gon Freecs VS Izuku Midoriya/Yusuke Urameshi

Here are some non-anime fights though:

Ness (Earthbound) VS Silver (Sonic)

Ultron (Marvel) VS Sigma (Megaman)

White Ranger (PR) VS Leonardo (TMNT)