It all depends on what you want. I also like Silversea because I have had some wonderful experiences on the line - especially with their outstanding shore excursions.
Regent also specializes in shore excursions - in fact they include them in the cruise fare - so I would also have no problem recommending them.
In terms of sheer shipboard experience, I personally prefer Crystal because of its abundant onboard activities. I hear people complain about the smaller staterooms (they are older ships) but I find them to be extremely comfortable.
I like the look of Regent's itineraries - packed with many ports of call. Crystal tends to have fewer days at sea - because their customers really enjoy the onboard experience.
The one type of "luxury" cruise I do not enjoy much is the very small ship (lots of motion of the ocean) on itineraries where you rarely get off the ship. This is OK for people who don't mind the solitude and remote feeling of being at sea with limited communication with the outside world, but to me it feels more like a prison.