About a year ago, I switched my list from MAL to AniList (due to a dumb announcement from MAL which I don’t even remember).

As for animeplanet; heard of it but never used it

Some differences between the two (from the view of AniList);

• AniList has a LOT of tags, which are voted on by users. You can search for spesific tags and add a minimum tag-vote% to your search.
• AniList doesn’t list Openings/Ending
• AniList has less reviews compared to MAL (like 39 vs 337 on Takopi's Sin)
• AniList related entries feels less organized (but I was already using chiaki.site to find all related episodes/seasons)
• AniList excludes/misses some spesific episodes, such as; recaps, manner movies, AI-generated (see Twins Hinahima)
• For me, AniList has a nicer UI, but I guess that’s subjective so decide for yourself

Despite some missing features, I do really like AniList. I also have AniList Extras(=userscript), which adds some MAL features to AniList (openings/endings, MAL-rating, hyperlink between the MAL/AL page). AniList is now my main source, but I also check the MAL page sometimes.

I also saw you were asking about finding new shows in another comment, so I’ll finish off with my method for that.

• Searching for spesific tags / tag-combinations / genres / …
• On AniList/MAL, each show has user-voted recommendations, so check those on shows you liked
• Check out the studios/producers for shows you liked
• Scrolling thru the most popular shows (either by rating or members) if you’ve not watched that many popular ones
• Lastly is just checking out random anime you see; be it from fanart, memes, OP/ED songs you hear

I’m quite sleepy rn, hope what I wrote is readable x)