Re: How many plug-ins do you consider essential?
Nah, I find that linear EQ's have a certain type of clarity for treble boosts that other EQ's don't always have. And certain bands and hopefully adjustable Q's for graphic EQ's are more useful than others for comb filtering drums. And not all EQ's can do Mid-Side as easily as others. And some EQ's have good metering, others don't. And EasyQ makes it really easy to sweep and listen to get really precise, but it can't be automated, while Reaper's EQ can. And then there's Linear Phase Graphic EQ which can do 96 dB cuts and some exotic curves. And some EQ's are good at boosting around 13, 16, 18, 20 kHz but others sound harsh. And then there's Dynamic EQ's which rock!!! And then there's the EQ's built into certain limiters which have their own unique sound if the limiting is on.
Specifically, EQ's have different amounts of phase shifting and their user-friendliness can vary alot. One of my favorite EQ's is the built-in EQ within MultitrackStudio Pro Plus, but it clips if you push it too hard (the engine isn't totally floating point). But it's got a great input/output meter and you can visualise the EQ curve really well and specifically get what you need if you stack a small variety of them.
Terry West's CS12M has a really nice bass and treble boost which isn't very versatile, but it sounds good on certain material. And the NYC style parallel compression (which involves EQ) sounds really good in some circumstances. I know it's compression, but it does have a tonal characteristic which is why I use it. TLs Maximizer also affects bass a particular way with one of it's settings.
So yeah, it's easy to get carried away, and I usually use a variety of these on both individual tracks as well as on the master.