Ssl, like already said, great for in you face, punchy sounds. I like it for snare, specially with the compressor on top. Not that good for cleaning up.
Ren, good for much more detailed dips and I like it also for smooth boosts in the mid and high freq area. For example, boost 8k/ 12k on vocals, or a more soft snare sound. Its a pretty smooth eq.
Q10, I find it a bit harsher than the ren. Not good for smooth boosts, but very very good for super detailed cuts ( the q factor on those plugs goes insanly narrow) and maybe boosting for roughness, say bass, or if you want to ruin something....
APIs very good too. Not detailed at all, mid to large paint brush. Killer on drums. Good presets, as starting points.
+1 on that. I too find it a bit hard, almost like ... dunno ... nasal.
And as for the post above mine .. if you're looking for a general usage EQ, tracking EQ, the difference between EQ isn't that dramatic. There was even a thread here not too long ago that claimed that you can emulate the effect of every EQ, with another EQ, most EQs in fact, no matter what price or pedigree. Regardless if it's true or not, it's still a very relevant point. The functionality, flexibility and how immediatly comfortable you find yourself with an EQ, is still just as important. I've tested myself many times, using different EQs and adjusting the same track with them. You'd be surprised to know how much the GUI controls how you tweak your EQ, and I tried very much to let my ears guide me. But my ajustments were different from EQ to EQ and I think most of that was because of how the GUI made me hear things I really didn't hear etc
The q10 is more surgical than the Ren series.... though in most cases I'll opt for the Digi Eq 3 over either of these for cuts and corrections. I find the Ren eq smears things quite a bit, depending on the source. I hate it on crunch guitars. Boosting with either of those is a nightmare to my ears, especially the treble regions.
The REQ. was co-designed by Hutch of Manley at the same time the Massive Passive was being designed. I always thought they have a bit in common sound-wise. Great for some things and not so great for others.
REQ was the very first double precision EQ plug-in back when DSP economy was absolutely critical. The fact that lots of people still use it after more than a decade speaks volumes.
The Q-10 was the very first plug-in. I still use it from time to time at 96k which avoids the problems mentioned.