Which idols will have the biggest loss when their group activities end?

Some idols are better off going solo, some are better in a group environment.

Which idols, when taken out of the group context, will suffer the biggest loss?

Usually it's the least popular members of a group who lose out at the end. Their fame gets hard carried by the popular members and when it's just their own small group of stans, they eventually flame out.

All of Twice except Nayeon.

And MAYBE the J-line in Japan.

twice members tbh and thats on jyp .. if only the company let them do solo activities they would be known to the public and popular overall by now, i know they started promoting recently but i feel is too late.

The unpopular ones

The vast majority. Solo career isn't easy