I am not an expert in ez mix, in fact I just did the same thing you did (except I bought the guitar god pack + Ez Mix 6pack ), So I am still working my way through all my new toys but here is my pick 6 and I am mostly metal too and spent way too long making my picks

I use superior drummer with + Metal Foundry & Metal Machinery (Also have music city that I got on black Friday).

I have
Mastering I & II – not used much yet but I think it is a must buy (I just can’t personally confirm)
Andy Sneap – very cool but so far only played with the guitar tones
Colin Richardson – haven’t played with it much but I seemed like a good hard rock sound which I also do.
Metal – Hightly recommended, love the drum presets (this has to make it into your list)
Metal Essentials – So far so good, not as good as metal for me at least for the drums
Guitar Gods I,II & III (got in a pack with ez mix w) I think I like II the best but I have had these longer and used them more but some very good sounds here [note: I have to adjust the input volume on most presets to make the gate work correctly]
Classic Amps – got this on the 5 days of Christmas sale, probably my least favorite of the bunch, nothing metal or even hard rock here

By the way, I also own Amplitube 4 with almost everything from the custom shop and I have been happily using just ez mix for the last couple of weeks. Not saying it is better but I want to focus on recording and ez mix seems to make that easier right now and the sounds with ez mix are perfect for flushing out ideas.

I have my go to tone in amplitude but having all the pre-made sound in ez mix has given me a lot of ideas, I ended up liking things I didn’t think I would like. I don’t really have to spend any time tweaking. In amplitube if I start changing my sound I know it going to be an hour or two of messing around with tone (and I won’t even like my original tone when I am done) so I just don’t change my tone often. It’s weird there is a benefit to the take it or leave it tone in my productivity.