I use Waves plugins but I've had more instalation problems with them than any other plugin maker.
 
These make it onto just about every project
J37 Tape
NLS 
SSL-Bus Comp
Abbey Road Plate Reverb (uses a ton of CPU though and usually only on a track or two)
 
 
Schepps Omni Channel is probably the best bang for the buck plug they have when on sale for $29.  If you are going to get one, that is probably it.  Saturation, EQ, Comp, etc and they play nice together.  Looks simple but there are deeper options under the hood (check out some videos on it).  
 
Also note if you use more than one computer, Waves is one of the only company that gives you only one single liscence.  I HATE that.  You can author to a USB stick, but then you have to use a USB port just for that.
 
They make pro quality stuff, but I seem to have trouble with them every single time I try to add another one to the collection and the Waves Installer.  They also take a LONG time to scan on startup, so if you buy them, you might want to check the don't scan at startup button and just run it when you add more to the computer.
 
Cakewalk's quality will depend on what you have.  Things like the EQ, MB Comp, Limiter, B-Reverb, Rematrix Reverb, and CA-2A are all top shelf.  (you might not have some of this unless you had Platinum before Bandlab).
 
Depends on the effects you are looking for.  Don't get trapped into thinking Waves is "it," but they do have quite a few useful tools. 
 
I use Izotope a lot (Neutron, Ozone, RX)
 
Boz Digital (Mongoose used on every track with Bass)  
 
Toneboosters had good quality for cheep
 
If you have Platinum, there is likley nothing you actually need (other than noise reduction).  There are are just different flavors and workflows by other manufactures.  Example, I could use the Tape Sim in Cakewalk, but find the J37 does sound better to my ears, same with the NLS vs Cakes'  But these are subtle color things and in an blind test, might not even pick the right one.  

As for the original question regarding quality, you'll hear very few complaints about Waves products. Aside from a few UI issues over the years, they have been remarkably reliable. In terms of audio quality they are as good as anything out there.
 
The problem with Waves isn't with quality, but rather their business practices. After three separate occasions when my Waves plugins abruptly stopped working and essentially accused my of being a thief, and (at that time) there was zero support available without a WUP contract, I abandoned Waves.
 
Once upon a time, Waves offered unique products that had no comparable substitutes. The good news is that that is no longer the case. These days folks can easily equip their toolkits with everything they need without Waves.
 
Assuming, of course, that one's bag of tricks is actually incomplete to begin with. I would argue that with X2 you already have everything you need, all of excellent quality, with just one notable exception: X2 lacked a really good limiter. Get yourself a copy of Ozone Advanced from iZotope and you'll be set.