Well…
1. z Grid would be my first choice as it will have a very wide impact on plugin usage overall in Podium.
After that…
2. z Compressor
3. z Delay
4. z Chorus / Flanger / Phaser
5. z Sampler
6. z Filter
7. z Dither
That may cover 2 years of development, but I thought I might as well list them all. 🙂
I think as my personal preference, probably in that order (definitely zGrid first IMO) but that is just my preference other users may of course have a totally different list 8)