Tough question... I could probably write a book<G>!
I divide the plug-in world into two broad categories - those that try to emulate 'classic' hardware, and those that try to go beyond the old hardware.
To my ears (and this is a very personal choice) nothing comes close to the Universal Audio plugins for emulations. Yes, you have to buy a DSP card, and yes, it ain't cheap, and yes, you will then be limited by that DSP card, but it is still well worth it - to me - when I want to use something that sounds like the hardware I grew up with. Also in the category are the delays from PSP Audioware... their Lexicon PCM-42 is amazing! There are others as well, but I'm happy with what I am using now.
The other category is a LOT more complex - there is a ton of stuff out there. I'd start with the plug-ins that Cakewalk provides with Sonar. I'd then look at companies like Voxengo and Waves. The Waves "C" family of compressors is among the most flexible I've tried. You can do nearly anything with them. I'm also a big fan of Voxengo Crunchessor.
I don't know if you can still download them, but the entire Golden line from Kjaerhus remains at the top of my go-to list. These plugins sound great and are nearly as flexible as the Waves plug-ins.
Falling somewhere in the middle, the SoundToys native suite is amazing. I've used them for mixing, and for sound design work. They are much more flexible than your typical emulation!
Fab-Filter is another fantastic developer, and while I still miss my copy of Antares Filter, I am learning to get the same sounds from Fab.
Recently, as a result of their Christmas sale, I've become very interested in several of the lines represented by Plugin-alliance. I'll be grabbing a couple of new plugins from them before the new year! Embarrassingly enough, one is in fact an emulation<G>!
I think that if you did not grow up using 1176s, Pultecs, 160s, etc then you probably don't NEED emulations. You can get a lot more mileage from the non-emulation approach. You will not likely ever get the exact sound of an 1176 with C1, but you'll get very effective compression. It seems to me that one of my biggest weaknesses is that I still tend to reach for gear that I know really well!