I really wish there was a single, perfect answer for this, but the landscape for free Japanese webtoons with consistently top-tier art is honestly pretty fragmented. The term 'webtoon' itself often gets used interchangeably now – you have Korean originals, Japanese manga adapted into vertical scroll format, and sometimes original comics made in a similar style but from different regions. If we're talking specifically about Japanese comics in that vertical, full-color format with art that'll blow your mind, the pickings on completely free, legal sites can be slim.
Your most reliable bet is often to look at the official sources that offer free first chapters or rotating selections. Manga Plus by Shueisha is a stellar example. It's not a webtoon platform per se, but it hosts series like 'Spy x Family' in a vertical, scrollable format with official translations, and the art quality is, of course, pristine because it's straight from the publisher. They have a huge library where the first and latest few chapters are free. For stuff originally conceived as digital vertical comics, Comic Walker (by Kadokawa) and Nico Nico Seiga have sections for 'vertical scroll manga'. The art quality varies by creator, but you can find some real gems with stunning, detailed panels that use the vertical space in creative ways.
A word of caution on the aggregator sites that pop up first in searches – the ones with names like 'mangareader' or 'mangakakalot'. They do aggregate tons of content, including colored versions of manga and Korean webtoons, and yeah, it's all 'free'. But the art is frequently compressed, watermarked, or scanned poorly, which absolutely murders the detail and color work. The translations are also a dice roll, and the whole experience feels a bit like browsing a digital back alley. I'd only use them as a last resort to sample something obscure, not for the art appreciation experience. The real high-quality view is usually locked behind the official apps and a few ad-supported legal portals.