I've stayed at all 4 in the past 12 months, with multiple visits to most. I would say each is great for various reasons. What is the context of your stay? Is it a weekend, is it a family trip? Are you coming specifically to go to a show or some other event? FYI, fantastic weekend rates (~$100-$150) at all 4 can make using points a poor value.

The Allegro and the Gray are both firmly IN the loop and would make for great weekday stays, but poor weekend stays, for the most part. The Palomar is in River North and a better fit all week, while the Monaco is close enough to the river to work for both as well.

The Gray is very recently refurbished and quite nice. The Palomar is right behind it in terms of age. The Allegro is a little dated, but I think it's got a ton of character and still pretty nice. The Monaco is probably in the middle. I've had great service and significant upgrades along with good IC welcome amenities at all 4. Also had good experiences as an (old Karma program) level 2 and 3 over the course of the year. If you can provide more details about the nature of your stay, it would help focus your search.