Since quite some time there are 3 different groups scanlating and releasing kenichi at almost the same time, I was curious if anyone has opinions which group has the best overall quality? (translation/cleaning/raws/etc)

I'm actually on a hunt for high quality HSDK content at the moment so I may have some insights.

Mangastream did HSDK for awhile and it was one of the best. Keishou is shutting down apparently - so they aren't an option anymore.

Currently Red Hawk is doing a joint release with Muda Scanlations and IEatManga - those releases are under the 'MRI' moniker (their names) and they are pretty much the best I have ever seen in all categories you mentioned. The downside is that due to various issues they are not going to continue it for more than a couple weeks.

If Muda Scanlations continues scanning it after 'MRI' stops then they will be the best IMO.

The remaining groups (Utopia and DiscipleScans) use the same translation source and raws but I think Utopia has better cleaning. However, the images done by DiscipleScans are a little larger than Utopia, not much though.

tl;dr: When the dust settles:

1) Muda - best overall if they continue doing it, cleaning, TL, etc.
2) Utopia - good cleaning, alright TL, probably the one I will use if Muda stops
3) Disciple - alright cleaning, alright TL (same as Utopia), slightly larger image size

*As always, support the author by purchasing their works when/if it becomes available in your country etc. etc. etc.