I have not been on Regent - I was on Radisson in about 1996.
I think Crystal has the best food on a cruise ship I have ever had - although the best lobster I ever had was on Princess in a specialty restaurant.
Silversea is very comfortable - not stuffy. I love their tours and the way they treat, especially on tour. I am not ahuge fan of their European cuisine - don't get me wrong, I think it is superb, but it jusdt is not my personal taste (fois gras in aspic, for example).
The staterooms on Silversea are the best afloat IMHO.
I have not tried Seabourn either.