So after some time in Rome, my overall ranking:

1. W Rome (phenomenal service)
2. Le Meridien Visconti (best access to neighborhood restaurants, less touristy)
3. St. Regis (luxe rooms, proximity to train station and tourist attractions)

The service at the W is head and shoulders above the other properties I tried. I was blown away. It’s one of the rare properties I think I would go out of my way to book (i.e. ignoring budget considerations, etc.). I’ll post a more detailed writeup in the property specific thread, but for now, ElevatorEnthusiast you should consider returning to Rome for this hotel!