Get the Faith of our Fathers/Rock of Ages tins associated with the wanted theme. This is basically mandatory, with the exception of some new themes (Disciples and N.T. Prophets) and old strategies (old school speed, FBTN{B}).
Disciples is definitely the next thing to try as far as boosters go, as they give you a plethora of cards, and some good ones from both new and old sets. Lots of needed ultra rares (Guardian, Destruction, Unholy Writ, etc.) can be found in these, in addition to some very key new cards for your brigade.
TexP is also good, but not as good, in my opinion, because you only get two cards of the new set, so you're less likely to get something you can use. It also doesn't seem to have as good of ultras. Mayhem and Grapes are amazing though, and there are some brigades that need TexP (Orange, Black, and some others).
After that it'll be difficult to get what you really need out of boosters no matter what you buy. I personally like Priests, as they have some nice commons, and some really powerful rares and ultras.
Then Patriarchs, although you'll get a lot of these rares/ultras in Disciples and TexP, there are some really great commons.
Then Kings. Just because.
Then Prophets...because Isildur somehow makes them work...but I've never seen them to be effective in anybody else's hands.
Then Angel Wars. If you're desperate. Some nice rares here and there, but bad odds for them. You're better off buying/trading what you need outright.
Then Apostles. Everything you need from this set you should be able to get out of Disciples and TexP, with the exception of one or two cards that you can probably get for free (like Good Samaritan).
Then Unlimited. It's pretty much garbage, other than a few decent rares (2 liner lost soul, Authority of Christ, Snare, and Stocks...), but no necessary ones.
