For signal processors, I use most often the plugins that come with Sonar Platinum. For bread and butter plugins they are more than adequate. If you can't get good results from them it's almost surely a lack of skill that's the problem, not the plugins. For me, when it comes to things like EQ and compression it just takes a lot of patience and careful listening to get it close to right because my skills are middling at best. I think you can get pretty far without any third party plugins.
Having said that, third party plugins can sometimes a) offer a different tonal flavor, sometimes subtle sometimes dramatic, b) simplify getting certain sounds, and c) do specialized "effects" much more easily and simply than setting up a complex processing/modulation chain.
I've been picking up some Waves stuff lately as their sale prices have been pretty dramatic. Everything I've gotten either does something kind of specialized (Reel ADT, Abbey Road Plate reverb) or is a model of very specific analog hardware unit. The great thing about the latter is they are modeled on circuits that were designed to get specific musical results with much fewer options than are available today in software, which can be a good thing. I've been using the Scheps 73 a lot, modeled after a Neve 1073 console preamp/EQ section. I have no idea if it really sounds just like a Neve - I've never been near a Neve console - nor am I all hung up on "Oh it sounds so totally analog, dude!" What it does is offer a simple set of controls that have been very carefully designed to give musical sounding results in a straightforward way. Quad Curve EQ is great, I use it all the time. It can do virtually anything, including getting EQ settings very close to the Scheps 73 (also including making a next-to-intractable mess if you're not careful) . It would take some finagling to approximate the Scheps, and then I'd need some kind of saturation plugin to add some subtle harmonic distortion (or not subtle if I want to mimic driving the mic pre with a line level signal). There would probably be some difference in the way the different processes interact, but with some time and effort I could get something very close. But it's just easier and more intuitive to drop in the Scheps 73 and get it right there if I'm wanting that sound. Same thing with the Kramer Pie compressor. It just has a certain tonal flavor when I put it on a drum bus, a smoothness in the top end that I like. I could likely well get about the same results with Sonar's included plugs, but again this one just gives a particular tonal flavor very easily.
I will add though that I wouldn't have ever paid the original very high prices for any Waves plugins. What they offer just wouldn't have been a good value to me, but I suppose for those who could afford them, it was worth it. At rock bottom prices like they're offering now though, totally worth it.
Also, there are a couple of specific things that aren't covered by any Cakewalk plugins. One is harmonic/aural exciter, like the Aphex units. This is a tool I use a lot actually to give certain things that really shiny presence. Also mid/side EQing, which is a pretty cool way to EQ stereo busses.