I'll bite since I used to some mastering for clients who actually paid their bills.
First off, there really are no "best" mastering plug-ins and no "best" mastering hardware. For example I'll probably choose a Weiss over a Behringer for EQ but if my sonic objective is something that a Behringer quickly delivers why not use the Behringer?
You want to know what is good? Go visit mastering studios or look at their gear lists. Check out Gateway Mastering. One of my favorites is Brian Gardner at Bernie Grundmans. Keep in mind a lot of the gear at Bernies has been altered...actually most of it.
Check out the gear, but more importantly learn how and why it is used. I sat in on a mastering session years ago with a guy who is now a very well known name. He had most anything at his disposal for high end gear. For this particular track(s) even though he had 2 very nice hardware limiters, he chose an L-2! He looked at me, grinned and said for these tracks he thought it would sound best! He also made me promise to never divulge his name. :D
When it comes to the topic of mastering I'm continually amazed at the focus on gear instead of procedure. A sonically engineered room with high quality LEARNED speakers and user experience is so much more important than gear. Gear is a tool. You use it day in and day out and eventually you become intimate with that tool knowing exactly how it will process any given audio to achieve a desired outcome. You know the limits of every parameter on your tool and you know when to select that particular tool. Most ME's I know of stick with what works for them. They aren't continually A/Bing the best new gear...even if in fact it might be "better." For example this is why I still often use an L-2. I know what it can and can't do depending on the given audio. If you read the forums, the L-2 is old, outdated, and distorts like crazy. But those comments IMO really manifest user inexperience. Experience has shown me that you can't get much more out of an L-2 than 2-3dbRMS without distortion. But that's okay because most experienced ME's I know of get the majority volume before any signal even hits the L-2 . Some of the ways is through EQ, Mid/side processing, parallel sometimes even series compression, soft clipping, and proper gain staging. Know how and when to use these procedures, then feed your limiter for a little bit more. Your limiter will most likely reward you greatly if it's fed something good.
Most professional pop/rock mixes arrive relatively hot. I hate to use numbers because content varies so much, but just guessing -13dbRMS to -11dbRMS. This volume has been established before mastering. Most of the completed pop I did was in the -10dbRMS to -7dbRMS range depending on the content.
What you don't do is slap on any limiter at the end of the chain, crank it, and then complain how it sucks compared to your new favorite limiter of the month. These days there are so many fantastic new tools...even free ones like Limiter No. 6 that I think it's a waste to focus so much time on the tool instead of the process. It's an area of diminishing returns for your dollar when mastering procedure is so much more important. Also, at what point does audio that is claimed to be "better" become just audio of another "flavor"...not worse or better...just different? I have seen blind tests where audiophiles are outright funny spouting endless sound adjectives. I usually offer them more wine. ;)
Personally I like Wavelab, but since WL7 be ready for an education as it can take some time to learn. Wavelab is deep, but IMO the most flexible and powerful mastering application around. Use Sadie if you don't need what Wavelab offers. Can you master in Cubase? You can "master" in most any DAW. But what will you do for DDP, RedBook PQ codes compiling, error checking, analysis, and/or burning? What will you do when your client is sitting behind you and insists A is better than B when in fact they are biased by comparing a slightly louder track? This is why I use Wavelab.
What do I use ITB? Select tools from UAD, Waves, and a few others. I mastered an album a few years ago from Bruce Swedien. It needed...hardly anything. :D All I did was some dB's, very slight imaging and then the regular things for any CD. Every project is different. To believe there is some silver bullet limiter/compressor/EQ/multiband whatever that surpasses all others is futile and IMO often foolish. Choose one or a couple...and LEARN them.
You can also go to mastering forums and read a gazillion opinions about the best mastering tools. Unfortunately just over the past 5 years many well known ME's including Bob Katz have given up offering advice for very good reasons. :mrgreen: