RenEQ and RenComp are workhorses that always sound good to me. Anyone looking to get their feet wet in the Waves world without too big an up front commitment should look at the RenMaxx package, which includes all the Ren plugs plus the IRL and a couple others. It's a great value.
I have the Platinum, and to me that's the sweet spot. You get the most functionality for the best price there. Moving up from there you get some cool stuff but as with all things the cost goes up fast relative to the extra functionality.
IR-L is quite nice and I use it on every project as well as SuperTap and the doubler (as a harmonizer.) C1/Q10 still have their place if you want something cleaner sounding (the Ren stuff has a more vintage vibe.) C4
is a really nice multi-band, and L1, L2, and LL3 are all very useful. Enigma can be used for some cool stuff as well. For mastering there's the Phase Linear stuff.
If you have IR-L, be sure to spend the $10 and get the two DVDs of impulse files. It makes all the difference to have plenty of really high quality (24/96) IR files to select from.
The SSL/API stuff, as mentioned, is some of the best stuff out there today. For aggressive drums, the SSL is pretty awesome. For more laid drums I'd use RenComp/RenEQ instead. I demo'd the API and it's great and I'll get it when possible.
I always find myself coming back to the Ren comp and EQ. Yeh, the SSL bus comp sounds nice, but throw RenComp on the master bus with the right settings and it's may sound even better. For syrupy/slinky guitars, the comp in Opto/Warm mode is really nice.