There are some excellent cheap (or free) quality plugs I am fond of. Look at the Klanghelm ones, ACON Multiply, Tokyo Dawn Labs Nova (dynamic equalizer), Kotelnikov (compression), SlickEQ, and Proximity (includes an ADT setting). It's easy to experiment with these too without busting the bank or getting overloaded with too many choices.

Echoing many users here...the stock plugins that ship with Sonar are great and can get the job done.  I will say that I use the free DigitalFishPhones plugins (BLOCKFISH, SPITFISH) on almost every mix I do and everyone should check them out.  I also have some go to Waves plugins that I love.
 
Carl

Waves musician 2 bundle is a great inexpensive compliment to the Sonar plugins. It's not that expensive and can greatly enhance your music. Wait for a sale and get it for between $69-$99. I am not a fanboy but a frugal musician.

Thanks for all of the advice, it was great to see how different people make their decisions. 
 
I purchased Waves Gold many years ago and took advantage of a Gold/RenMax upgrade.  I still expect to reach for the cake stuff first, but the Gold/RenMax package seems to offer a fine alternative when needed. 
 
I think I've got things well covered now for any need, as well as some choices when I want one.

+1 On the Nomad Blue Tubes suite. I use the Auto Track Box, the limiter and the various compressors all the time and I really like the delay too. People say these plugs are showing their age and they may well be, but they sound really good to me, and good sound is good sound no matter how old the plugs are.