also check out zebra2, a real workhorse, and a +1 to synthmaster

Sylenth1
 
Lennar bundles this with '32 Lives' which is a bridge tool required for running the current version on a 64-bit Mac OS (native 64-bit support for Mac OS is in the works apparently).

Admittedly, I do use presets quite a bit and I just edit them to suit my needs. A lot of the synths I've designed from scratch end up sounding exactly like some of the presets anyway, so for me it's easier to refine existing ones. Of course there are always instances where there's no preset that achieves the sound you need, in which case you have to build it yourself. :)
 
So far I've tried Serum, and my first impression was that the interface is very pretty aesthetically and it sounds sharp and deliberate. The website calls the oscillators "clean," which I like. I'm a big fan synths that are inherently bright.
 
These are all great suggestions! Thanks, everyone. I look forward to playing around with all of them this weekend.

Splat
Try NI Monark, reaktor, absynth, and even fm8.
Great sounds although arp functionality is almost non existent (apart from reaktor).

If you are using that and massive it's pretty much all you need. My big recommendation is to get away from the presets, groove3 tutorials help a lot.

For best value reaktor wins hands down,the number of free synths bundled to download is mind boggling. Of course these are all available as part of a bundle.

Monark is fantastic although as a monophonic synth I wouldn't consider it a good replacement for Z3TA+2. Being a straight ahead Moog emulation it's also lacking in many of the features which make sound shaping so convenient in other synths, like synced LFO's, semitone oscillator tuning etc.

I would definitely go with Massive as a Z3TA+2 replacement. It has excellent routing and modulation features.

I've not worked with it, but I've heard it over at a friend's studio and I'd second Sylenth 1. From what I saw last week or the week before, they are finally in the process of updating it to 64 bit.
 
Like you, I do rely on presets heavily, though I do customize them a bit if need be. From working with it from way back in the day until recently, I sort of knew Z3ta like the back of my hand so to speak. It was my go to synth for some things and crucial to certain projects I've been re-working. So it stinks that Cakewalk doesn't seem to come up with a fix.
 
Same could be said of Rapture and Dimension Pro which were two of the most important 3rd party tools that I own. Amazing synths and a shame that Cakewalk dropped the ball. 

One of the things that make Z3ta unique is the wave shaping capabilities; version 2 of Z3ta allowed the wave shapers to be modulation targets.  The closest thing I know of lives in Zebra (now at version 2.5 with a new release promised at some unspecified point in the future).  Zebra has something similar to Z3ta's wave shapers, but it also has much, much more going for it.  Zebra is a top-five-favorites synth of *many* musicians.  I'm not saying it sounds a lot like Z3ta (actually much better, IMO), but it has some similar synthesis capabilities.

Like others, I would also suggest Massive, but you already have it.  What is it you do with z3ta that you feel cannot be done with Massive?  The other alternatives have pretty much been listed as well. I think sylenth is the 3asiest to program of the bunch.

I wouldn't say I'm an expert with Massive but I do know how to build sounds from scratch. There's a certain kind of saw timbre in Z3ta (found in a preset called "Teccnoe") that, for the life of me, I can't figure out how to emulate with anything else, including Massive. It's very sharp and buzzy. I could list a dozen adjectives and I don't think it would matter unless you've heard it. I'm particularly fond of this timbre and I use variations of it in almost anything that involves synthesis.
 
I'm using Serum right now to try and recreate the timbre. I have to say, if you haven't tried Serum yet, it's pretty fun to play with. I'm already addicted to editing the wavetables and I'm getting close to the Z3ta sound I'm looking for. Still haven't perfected it yet though. I think this is the closest I've come yet.

Over at kvr there's a thread about things getting solved for LPX by CW.
In the meantime, regarding saw timbre sounds, have a look at ImageLine Sawer. I got to know this one thanks to Mesh. You may be able to get it at Musician's Friend for a low price, under $30.

exbehave
I wouldn't say I'm an expert with Massive but I do know how to build sounds from scratch. There's a certain kind of saw timbre in Z3ta (found in a preset called "Teccnoe") that, for the life of me, I can't figure out how to emulate with anything else, including Massive. It's very sharp and buzzy. I could list a dozen adjectives and I don't think it would matter unless you've heard it. I'm particularly fond of this timbre and I use variations of it in almost anything that involves synthesis.
 
I'm using Serum right now to try and recreate the timbre. I have to say, if you haven't tried Serum yet, it's pretty fun to play with. I'm already addicted to editing the wavetables and I'm getting close to the Z3ta sound I'm looking for. Still haven't perfected it yet though. I think this is the closest I've come yet.

I think I know what you are talking about . This quality of z3eta is sometimes used as a knock on it. I suspect the difference may lie in the waveforms. See if it is possible to import the wave cycle used in z3ta to massive. It may bring you closer to the sound.