My anime rankings include 178 shows, 81 of which are inspired by manga. That roughly means that your good manga ranking should be half as competitive as your good anime ranking. So a manga ranked 10th, for instance, you would expect to be ranked 20th among anime, all else being equal.
For some manga, though, this is far from true. The rankings between the manga version and the anime version are a gulf wider than the Pacific Ocean -- if there's any ranked anime version at all. It's at times like these that I hope people aren't fixated on anime alone and branch out to enjoy these truly marvelous mangas that they'd never learn are any good through watching the anime alone.
Take the 4th best manga of all time, Negima/UQ Holder. One would expect it be ranked 8th in the anime category. Instead it's ranked 155th.
10th best, Akame ga Kill, should be 20th in the anime category, is instead 98th.
Freezing is the 11th best manga of all time, so that should be 22nd among anime -- it's not even ranked.
12th best is Akatsuki no Yona, so that should be around 24th best in anime -- instead it's 131st.
16th best is To Love-ru, so that should be around 32nd best in anime -- instead it's 100th.
31st best is H2, so that should be 62nd in anime -- instead it's 162nd.
32nd best is Zettai Karen Children, so that should be 64th in anime -- it's not even ranked.
33rd best is Btooom!, so that should be 66th in anime -- instead it's 148th.
34th best is Vinland Saga, so that should be 68th in anime -- it doesn't even have an anime adaption.
One Piece is my 13th favorite manga. Behind Akatsuki no Yona, Freezing, Akame ga Kill and Negima/UQ Holder. If you think One Piece is good, just wait until you read these mangas. They don't have any of the inconsistencies, annoyances, or do-gooder no killing rules that plague One Piece. It's like you're finally stepping out into the adult world, where the fiction actually mirrors reality, and is thus infinitely richer and deeper a story as a result.
I'm very happy that One Piece is being animated in full, with over 600 non-filler episodes to enjoy. But if any tiny portion of that budget had instead been spent to adapt the remainder of these other mangas, or even to adapt these stories for the first time since some haven't even been touched yet, the world would have been a much better place. People have no clue how much they're missing out on. If I could take the (eventual) 1,000 episodes of One Piece and redistribute them freely to other projects, and no one got to see One Piece at all, can you imagine how much artistic profit there would be? The number of other shows that could've been given their fair chance, that could have seen the light of day, would be astronomical. It's a shame anime is driven by market forces instead of enlightened billionaires who just want to promote good art, like in the days of Michelangelo. But so it goes, and thus we suffer.