Monitoring and room treatment trump everything -- E-V-E-R-Y-T-H-I-N-G -- so that's the obvious place to start. After that, the AD & DA should be considered (not counting the ultimate DA controlling the monitoring chain) before bothering with any particular outboard gear.
What's your current situation in that regard?
AFTER those are all handled, I suppose the next obvious piece would be a compressor. Some would argue EQ and they shouldn't be ignored. I've just found many decent digital EQ's over the last several years. But no substitute for GR (and additional gain) in the analog realm (assuming, of course, that the DA & AD are up to the task and the difference can be easily noted through the monitoring DA and chain).
*Which* compressor is open to debate. Personally, the only one that gets used on --- really, just about every single project that comes through the door, is the Crane Song STC-8M. Followed by the Variable Mu (probably on 10-15%) and then the goofy stuff like the SSL (maybe on 1 or 3 out of 100 mixes). That thing (the STC-8M) is freakishly flexible, amazingly clean - Very powerful unit.