What are the differences between cruise lines and which ones do I prefer?

Not all cruise lines are alike and some do a better job at customer service, cleanliness, overall condition of their ships, safety and comfort than others. Also some actually welcome and have decent sales on solo travel.  Based on the above criteria, below is my ranking of the cruise lines that I have sailed on. 1 is the highest and 6 the lowest
1. Holland America
2. Princess
3. Royal Caribbean
4. Norwegian
5. Carnival - NOTE I will never go on this cruise line again.
6. Costa

On Holland America I have been on the Oosterdam, Eurodam, Zuiderdam, Veendam, Noordam (previous ship to the current one sailing in the fleet) and shortly I will be on the Koningsdam.  I have only experienced one customer service issue (an industrial fan running outside my cabin on the first night) and with a bit of a hassle they moved me to another same category cabin (I truly believe it was the Hotel Manager on the Veendam that was the issue) as everyone else working on the ship was great.  I don't really recommend the Veendam as its quite old, worn and with that comes issues with the condition of the ship.  All other ships were excellent. I have been able to get most of the cruises I have taken on sale so the single supplement (which is often less than other lines) has been significantly less.  Note Holland is always a bit more expensive than other cruise lines.

On Princess I have been on the Sun and Star princess ships. Both old but in good shape with more pools than the newer ships. Food in the dining rooms was probably the highest quality I have had on the various cruise lines.  Cabins were clean and comfortable. Note its really hard to get a good price for solo travel on this line, thus why I haven't taken many cruises with them.

On Royal Caribbean I have been on the Mariner of the Seas, Jewel of the Seas, and Grandeur of the Seas, with next Spring a transatlantic on the Oasis of the Seas.  While everything has been OK on these ships for me, I find that they have the slowest dining room staff at sea.  It will take you the longest to get thru dinner and they tend to put solos at the worst location in the dining room.  They also don't do what I call pairing - similar ages etc. for dining  I have been placed with families with young children twice so requested a move (which they accommodated).  The cabins except for the Mariner of the Seas have been regular size - Mariner is much smaller.  Mariner also had NoroVirus and an infestation of spiders. 

On NCL I have been on the Breakaway, Pearl(2x), Jewel (2x), Jade, Sun, Spirit and Star. For many years they were my favourite cruise line but there have  been many, many cabin issues, cleanliness, customer service issues. And honestly I find regardless of the ship the Hotel staff are always horrible to deal with - expect 3 to 4 tries to get help and an escalation to the hotel director for simple things like non-functioning toilets, showers, carpet cleanliness, broken cupboards etc.  I probably won't return, and there fall from grace is disheartening to me.

On Carnival - what can I say I was on the inaugural sailing of the Sunshine - lack of toilets to use, leaks, security issues for pretty much all of the passengers. They returned 73.3% of my cruise fare and verbally told me that they wouldn't return all of it as I ate food on the ship. Just beyond belief horrible to deal with both on the ship and thru head office.  Worst ship and cruise I have ever been on.

Costa - low cost, ship propulsion and air issues on both the ships Atantica and M? - so bad I can't remember the name of the ship.  The absolute worst staff  I have every came across at sea. 

Remember these are my personal ratings based on personal experiences on each of the ships.  As you can see I frequently sail and am likely one of the easiest solo passengers at sea. The front desk and staff will likely not hear from me at all unless there is something broken in the cabin. I think food is to taste so really can not give you opinions.  Entertainment is also to taste and most lines recycle the same comedians, acrobats and dancers as I have seen the same ones on multiple lines.  I like to use the pools and except for Breakaway which only has one tiny pool for adults, all lines had good pools.

Hope this helps