Or this delay or that reverb?  Sonar Platinum comes with over 70 Effects and ProChannel modules, and if you're like me, maybe you've purchased some other plugins along the way. 
For my part I tend to reach for the ones I've most recently picked up, but that's not very scientific.  It seems like it would be time consuming to try every option. 
How do you choose?  Do you have a set of "go to" tools you work from?  Do you have certain ones you like for bass, or vocals or guitars?
Mr. Anderton has pointed out that one could use just the Sonitus tools and get a good result.  What makes you choose something else? 
I have a ton of options to choose from, perhaps too many.  I know some must be better than others, but I'm having some trouble figuring which ones are best.
I'd be interested to hear how you choose what to use this over that.
 

just through trial and error, hopefully not to much error, but by using your various options, over time you will know, get a feel that x plugin will suit x situation. let your ears decide, for example certain compressor might shine on the drums, or bass, while another will shine on vocals, eventually you get to know. then there is other influences, ease of use, cpu drain and on and on which may influence your choice. sometimes you may just try another option than your usual and really like it for that situation, it may even become your fav for a while. i have had a habit of just reaching for valhalla room reverb for guitar bus, recently just threw on rematrix, had a play around and found i liked that, not that it was better or worse, maybe just different, but at that time, the project, the mood i was in, whatever, it ousted my usual pick, variety is the spice of life they say, although to much choice can sometimes be a curse. that's just my thoughts anyway, ymmv

You will have to use the plugins you have and compare them with others to achieve the results you want.   
 
For instance, select a compressor and compress a very dynamic vocal track.   Next, place another compressor on the same vocal track and dial in the same settings if possible.  See which plugin gives the the best results and tame the dynamics better.  To me, that would be a compressor to use in my arsenal until I run across a better compressor.

Learning which plugin is best for which instrument is a huge undertaking but I have found that if you will solo the track and step thru the different plugs you will find THE one that really does improve the feel and attitude and the sound of the track. Solo that strat track and listen to it by itself and you will clearly see the difference in what each plug does. To me, it is very hard to have all the tracks playing even in Dim mode and really hear what that plug is doing for each instrument.

Its not the tools its how you use them i discovered.

Hey Schwa,
Choosing an effect all comes down to coloration.  Practically every DAW now come with effects that can easily make great sounding recordings using only what they come with.  The secret is in getting to know what each one does and determine what to use by the function you need it to do.  For example, find out which EQ you have can that can high pass tracks that don't need 80hz like overheads and use that.  Find which delay you can use as a slap that allows you to time to tempo.  Find which reverb has a great sounding Hall preset and use that on your Room bus.  Simplify and narrow your choices until you become more familiar with your system.  Then you can go crazy with outside plug ins.  
I use onboard Sonar effects both through the ProChannel and inserts along with Waves, UBK, Slate, SoundToys and freeware on my productions and I'm still learning.
I could suggest you look into a course like the ones offered on Groove3.com to learn more about effects and how they're used.
Then the decision process becomes manifestly easier.

I don't think it matters all that much. There are some few plugins more suitable to certain use but most in my view are pretty interchangeable. A good example for this is the CA2A. It is a specialty plugin that is perfect for some things but not most things. I'm sure many will disagree though.
 
I believe that its more about perception and not so much actual sonic difference. The GUI plays a role in this too.  I do recall that some years back a certain DAW had God awful plugins. They have since improved their bundled plugins. Sonar has been top drawer for including high quality plugins.   

My criteria, in order of importance...
 
1. Ergonomics. I gravitate toward effects that are quick and easy to dial in. ValhallaDSP reverbs are a good example. Simple, functional interfaces that don't get in the way. Resizable UIs (e.g. Melda and Fabfilter) are a big plus. Knobs that respond to the mouse in intuitive ways. The ability to type in values. Controls that aren't hidden. Controls that are clearly labeled as to their purpose. Labels that aren't too small or low-contrast, that can be read in low light. No pink or lime green. 
 
2. Functionality. If you're going to invest the time to deeply learn some piece of software, you want it to be versatile enough that you won't need to learn too many alternatives. Ozone, for example, is an all-in-one mastering tool so full-featured you don't need anything else. Pro-C does everything a compressor can be expected to do, and also satisfies requirement #1. One-trick ponies have a place, but they have to satisfy #1 or #3.
 
3. Unique tricks. Sometimes an effect has an unusual capability or characteristic that makes it uniquely suited for a particular application. FabFilter Timeless isn't as simple to use as other delays, but its unique modulation features assure it a position on the first-string roster. Fabfilter Saturn isn't as versatile as other alternatives, but it has capabilities no other distortion plugin has. Meldaproduction's MSpectralDynamics does just one thing, so it fails #1 and #2. But that one thing that it does is special. 
 
Surprising things that are rarely a factor: sound quality and aesthetic appeal.
 

Hi Schwa,
One way to approach that question is to think about various fx, both hardware and software, the same way you think about microphones. For example, if you're micing up your D28 you could probably make it sound OK with your 57. I mean, enough time spent with positioning and then the right EQ and it'd probably be OK. But why? You have your SM-81's sitting right there. You've probably used them on Ac Git a million times. You don't have to spend time messing around with them, they just sound good. Now, do you use spaced or coincident pair? Well, if you think there's a reasonable chance of needing to collapse to mono, probably coincident, but beyond that, they have different sounds. You've probably mic'd up your Martin a ton, you know which mic setup gets you a particular result and that's the one you go with.
 
Now, apply that to compressors - yesterday I was doing production on a vocal track. It was a male singer, not terribly bright sounding, doing a kind of funk song with fast lyrics. I could have gone with my TLA-100 (an LA2A-style compressor) or one of my 1176's. From a purely technical standpoint I knew that the TLA-100 was a bit too slow to handle the pace of the lyrics and the associated transients. From a sound standpoint the emphasis on the upper-mids from the 1176 sounded great. I could have probably gone with the TLA-100, and with some tweaking and whatnot it would have been OK, but the 1176 was right there, so may as well just use it and save the headache.
 
For your question about "go-to" tools, the answer for me is "sort of". I have tools that I know and like for particular applications, and then another sort of general set of tools for general tasks. Again, back to the microphone analogy - imagine you have a guy coming in to record an instrument that you've never heard of....maybe a hurdy gurdy or something. You probably have one or two mics that, generally speaking, are pretty tough for stuff to sound bad on, so you slap those on some stands and start there. Maybe after you hear the first take you break out the SM-7 because you know it does a particular thing when used a particular way, and why mess around at that point? I think most folks get to a point, if they do it enough, that fx become sort of like that.
 
Hope that helps,
Dean

The QuadCurve's four different curves demonstrate how EQs can sound different depending on what they're designed to do. I've found the easiest way to evaluate EQs is to feed noise through them, and you'll hear the difference between, for example, what happens to Q as you change gain with different curves. I wrote a "Friday's Tip of the Week" (Week 59: Understanding the QuadCurve—Why Noise Is Your Friend) about using noise to help understand how EQ affects a sound.