I currently have the included EZmix1 and EZmix2, plus Songwriter's Tools and Mastering2.
Now I'm thinking of adding Vocal Toolbox, Lead Vocals, Mastering1 and/or Ambient, primarily for singer-songwriter and laid-back orchestral stuff. Also using Alloy2, Slate and T-RackS in Logic.
Any suggestions?

The best thing to do is grab a 6 pack...and then 
BUY a 6 pack of EZMix packs...

Time+Space has it for about $110...pay in Euros for the best price

http://www.timespace.com/product/MMEZMP-120/6/EZmix/toontrack_ezmix_2_presets_bundle_%28serial_download%29.html

Already had one, Larry, different offer but still OK, now wondering which three to pick. Trying to help Mesh as well :)

Fleer
I currently have the included EZmix1 and EZmix2, plus Songwriter's Tools and Mastering2.
Now I'm thinking of adding Vocal Toolbox, Lead Vocals, Mastering1 and/or Ambient, primarily for singer-songwriter and laid-back orchestral stuff. Also using Alloy2, Slate and T-RackS in Logic.
Any suggestions?

i also suggest any or all of the vocal oriented preset packs. I've found it helpful when you are working with a new vocal artist. You can cycle through the presets an get a feel for what they like on their vocals. I have a Rapper that always for a particular preset now. I've got it saved as a favorite with his name on it.

The diff producer packs have a bit of everything. So does core expansion. They each have a list of the presets. See which have more of what you're.looking for.  If you're looking for Luxembourg Thrash, Polka, you are on your own.

BassDaddy
If you're looking for Luxembourg Thrash, Polka, you are on your own.

I expect that juan to show up around Oktoberfecks.

Allen ins Hofbräuhaus!
Thank you gentlemen, very kind. Anything on the Mastering packs?

Fleer
Thank you gentlemen, very kind. Anything on the Mastering packs?

Not much. For some reason I have Mastering II but not Mastering. Sadly, I've not used it. So I have no review of it.

Cheers Ed !
I think I'll go with the two Vocal packs like Dave said, maybe add Bdad's Core Expansion. Polka Style!

The Mastering packs are real good. Pretty much for mastering though. I think most of the packs have some mastering presets. The ones you picked should work great for you. I don't have any metal or the Electronic pack. The newest one, Lo-Fi, is the most limited one cuz wgo needs something like that very often?