Chevy
I'm on a tight budget, but want the absolute best sound I can muster up for my mixes (or masters).
Something that I could go to and confidently know that it's doing its job very well, that many others are having very good success with ?

The first i wanna say IMO:
There's NO best plugins for sound, but all have their "character" weakness, strength, in various areas.
A person need to really learn how to listen for the weakness and strength of those plugins.
Less is more... ( if you find the right ones ), therefore it is very important to know the sound and the plugin
to be able to pick out the ones you want later that fits your taste of sound.

If you have many plugins it will cause confusion, and you Will spend way more time to search trough
all the plugins you have to see what fit's best, and maybe you, like me just pick one and trow into the track.
well that said...

What do i mean by that :

EQ vice:
Many plugins will " dull the sound, by just inserting the plugin into the track, without adjusting it
It may add a more on 200-500hz, maybe decrease the high's. All this is very subtle changes, but you can
hear it when you is aware of, it and listen really carefully, by activating and deactivating the plugin only.

Stereo width:
Some plugins may be neutral, some will make it sound more mono, others will widen the sound
but as i said, very subtle changes.

Compressor:
Usually also have the above statements, and it is also how the way it compress the signal too
Is it quick, and like a smashing sound or do it compress in a soft way.

I own plugins from :

1. Waves ( a bunch )
2. Slate digital ( VMR,VTM,VCC ) and Revival
3. Izotope ( Ozone 6 and Alloy 2 )
4. PSP audioware ( PSP oltimer ME )
5. Focusrite ( midnight bundle )
6. stock plugins in Sonar Platinum, and the channel strip.
7. Eiosis AirEQ
8. A very, very few freeware plugins.

I certainly could had saved me $ 2 k worth of plugins, if i did not have the G.A.S syndrome
yes i admit it.... i do i do.
I regret buying so many plugins, while i could manage with way less.

It is almost a nightmare.
Don't get into the trap, that i did.... 
To tell you what plugin to buy ( for the best sound ) is very hard..
we have different hearing and taste and how it should sound like.

But it is possible to Mix and Master, great, all with freeware plugs, for sure.
Some free plugins :
1. Solid4010 Its layout and parameters derive from the SSL 4000 console
2. W1 Limiter W1 Limiter is a clone of Waves L1, with identical output
3. Bionic Delay separate control over the delay times on left and right channel.
4. Abstract Chamber Abstract Chamber is a super-simple reverb plugin, a natural yet abstract spatial impression.
5. FreeverbToo studio-qualitiy reverb, adjustable gate for gated reverb.
6. La Petite Excite small exciter plugin and it can lead you into a brighter future when used wisely.
7. Bark of Dog Bark of Dog is a bass resonance filter that lets you increase your low end without turning up the flab.
8. A1StereoControl A1StereoControl plugin you will be able to expand, or limit the STEREO WIDTH 
9. Tokyo Dawn labs free EQ and track compressor and Master compressor.
10. Klanghelm DC1A  compressor, DC1A is heavily program dependent, so is the saturation. 

This should get anyone what they need, for full mix and mastering for free, with great results.

My advice is go slow on your money for now, listen and learn a few plugins really good
then decide later if you need something else, then you know exactly what you like and
looking for.

Then DEMO it, real carefully... don't be to quick and buy.
Everyone wanna sell, it is how it's today, SALE here and there
tempting people to buy stuff the actually do not need. 

Shall i give an advice on one plugin i really is satisfied with, that would be the Izotope Ozone 6.
it cost some, about  $199 i think, but it is definitive worth it, me thinks.

Anyway hope you get something out of what i have said here.

All the best :

Jan

I am on a tight budget, but Nectar 2, Ozone 6, EZD2, and EZKeys  are what I feel I need to have around. They not only come with "ballpark" presets, you can learn by doing without getting carried away.  Oh, and the upgrade to Melodyne Editor.
 
The rest can be handled internally...... for me.

Chevy
Hi guys,  boy, what a learning curve this is!
I'm on a tight budget, but want the absolute best sound I can muster up for my mixes (or masters). All I have plug-in wise is what comes with Sonar Studio X3 /Producer 8...  the BC group, Nomad Factory stuff, LP-64 and misc, Lexicon Pantheon Reverb, and the stock Cakewalk and Sonitus offerings. Also a few stock 32-bit ones from Focusrite that came with the 2i2. And the Prochannel ones are there, too.
Since I don't have any other plug-ins to compare with, is there a decent package (or individuals) I could/should purchase that you would heartily recommend over the stock options I have? Something that I could go to and confidently know that it's doing its job very well, that many others are having very good success with ?

hi i have two names for mastering

Voxengo Elephant 4 with new algorithm and pretty cheap
but there is an awesome plug for mastering and many other stuff Sknote Soundbrigade
It's a new concept plugin and i have a fantastic result for mastering work.
Best

As mentioned, learn what you have, before chasing the next "breakthrough" plugin. When you find yourself limited with what you have, then go window shopping... Then you demo stuff. If you use the "new toys" more than your "old ones", it might be worth the upgrade. But do not let it hold you back. It is a long chase for the perfect plugin, I would believe (demoing most things I could get my hands on, through the years). Still mainly use PC modules, and bundled plugins within SONAR as default. But, hey... I am just a guy who loves to make music.
Others will certainly know better 
Best of luck.

Just to clarify - Waves no longer require iLok

IMHO the artist series (when on offer) are a great way to instant results with some high quality plugins

However, there is no substitute for learning the tools you have - and then the "gaps" or "I need a better..." will become obvious

Chasing a good sound via plugins is an expensive rabbit hole you need to think carefully before jumping in

My personal recommendation would be to get your head around all the tools in the PRO CHANNEL - the eq is great

The Sonitus compressor is also a really good place to learn about compression - you can also play with side-chain compression

First of all you need a good reverb, because their quality affect the sound more than eqs or compressors. Good chorus, delay and similar effects also important but less than reverb. I will not advise something specific, because it's a matter of taste (and money), but you will find some options on forums.

I would believe that genre of music is also a thing to have in the equation, regarding what you are looking for (what music styles do you do?). There are a few favorites in different styles of music. I would still not let it hold you back, in your search for "your sound".
But other do know better

EDIT : Deleted

SONAR provides most of what you need, even 8.  The ProChannel stuff is fantastic for meat and potatoes 1176 and SSL Buss emulation, as well as EQ.  Nomad provides the delays etc. effects I use.  Reverb - the new Breverb and convolution reverb are a nice step up - better than the old algorithmic Cake inclusions  and Perfect channel, which makes upgrading a good option.
 
The Mastering stuff is up to snuff, tho like above I use Voxengo Elephant and EQ for that job, more from familiarity than anything else.
 
Either upgrade to X or find a 3rd party reverb and maybe effects and you are good to go.  Then figure those tools out before you start spending big money.
 
@

Bristol_Jonesey
You can get stellar results with just the bundled plugins provided you know what you're doing with them.
 
Learn what you've got first then identify what you need.

Yes and yes . True that !