In my experience the ONLY line that truly tries to make the experience easy and fun for the solo cruiser is NCL. They devote one activity staff member throughout the entire voyage to organize a one hour daily meet up and make group reservations for solo diners in the MDR and at the shows. The solo lounge is also a great place to meet other solo cruisers in a relaxed atmosphere. It even sort of makes up for the tiny solo cabins.
You can get lucky on Princess and on Cunard by finding your own group through your MDR table mates or by finding one or more fellow passengers who take it upon themselves to organize the group dinners which really create the opportunity to get to know other solos. Princess probably irritates me most because they assign a staff member on only the first night to “host” a solo gathering, but the host disappears 15 minutes into the gathering and is never seen again. You may get lucky with the people you meet that first night and find congenial travel mates, or you may meet only people with whom you don’t care to spend 15 minutes, much less dinner every evening of a cruise. I’d rather eat alone with my book in the MDR than try to make strained conversations with people with whom I have nothing to talk about. But that does not make for an enjoyable cruise experience.
The cruise lines need to wake up to the fact that there are a lot of solo cruisers and we want a better experience than what is out there right now.