What would be the most important color for me to use in my painting?
How short should I cut my hair?
I need to season my food, which herb is best and why?
I want clothes. What should I buy?
To be, or not to be?
Sorry buddy, your question is just a tad bit vague, and any answer is just speculation. I do CD mastering, and I cannot imagine answering that quetion, pertaining to any collection of songs, without hearing the collection of song first. For me to say "Compression is most important" may NOT be with your songs, because your songs may not NEED compression. Same with eq, limiting, spacial enhancement, DC offset, etc......
Generally, "effects" are not used when mastering. You might use effects when you are mixing your songs. There are certain cases when a bit of effects MIGHT be used while mastering, but this is VERY rare. If anything would be used during mastering, it would be dynamic processors like compression, limiting, eq, and spacial enhancements. Some mastering engineers have some special tricks to add back dynamics (expansion) to mixes that are too flat, but they mostly try to avoid expansion and spacial effects if possible. In fact, a good mastering engineer will do as LITTLE as possible to the mixes. They know that anything they do can possibly destroy the sonic integrity of the mix!!! As one famous mastering engineer claimed once, sometimes it is what he DOESN'T do that makes all the difference! He felt that if the audio already sounds good, it is only his job to reaffirm that fact and leave it alone.
But, if the mixes need a lot of dynamic processing to attempt to bring the final mix up to loudness and sonic standards with what is currently the trend on major label releases, then they will have very expensive gear that does this quite well. MUCH better than anything you can get on a PC software program, or at Guitar Center....

They will also have VERY experience ears to tell when that processing is starting to adversely effect the audio in undesireable ways. Of course the audio can be processed well beyond that desireable point (listen to most modern hard rock mixes, that is what happened...

), but the GOOD mastering engineer would try at all cost to avoid doing that based on THEIR decision. If a clients wants their stuff overly compressed, overly limited, overly eq'ed, well, he will because he is paid to deliver what they want.
Anyway. If you are thinking of mastering on your own, just make sure to have a high quality compressor, limiter, and eq on hand. Which will do the best job for you totally depends on what your mixes NEED, NOT some pre-defined way of going about it. You need to have a good idea of HOW you want your audio to sound to be able to assess what processing should be applied, and of course, you need enough experience in using these processors over a music mix to know how to make the adjustments. Simply, there is no "standard settings" that apply. One song may need this, another may need that. It can be quite different from song to song.
If you are interested in professional mastering, there are probably 50 studios within a 2 hour drive from you (unless you live in the boonies) that offer it. How good they actually are is all in the ears of the listener!

There are also a few people who frequent this BBS that offer professional mastering services (yes, I am one of them, but don't think I am digging for a gig here....) that would be worth checking out.
Good luck.
Eddie