Which MTG booster packs have the highest rate of return on investment?
Anything after you wait long enough.

Of course, this is assuming you don't actually open the packs, but seriously, take a look at these prices:

$10 for ONE Lorwyn booster pack

$25 for THREE Time Spiral boosters

Over $100 for a Zendikar or Worldwake fat pack

Almost $20 for a Zendikar fat pack box with no cards in it

Since Magic is such a highly collectible game, people will tend to pay more for rarer items. This includes the booster packs themselves. Even though the cards from each of those sets are pretty cheap, the packs themselves are virtually impossible to find, so the price goes skyward. Sure, you could buy some Darksteel boosters and pray you get awesome cards and make a profit, but considering those are around $8 apiece now that's virtually impossible.

The other problem is actually finding people to buy your singles (you could sell in bulk, but that's going to hurt your profits even more). Considering that there's massive websites now that are devoted to nothing but selling singles and aggregate the results from tons of different sellers, you're going to have some tough competition. Even if people learn that they can buy cards from you, you'll have to at least match the prices offered on these sites if you want to stay marketable. You also have to have a LOT of cards available for sale, so get ready to buy 10 booster boxes so you can boast having all the mythic rares in a set (just like every other major card seller will).

The owner of our local card shop does a great business selling fat packs of sets that went out of print over a year ago online, and easily gets more than $80 for them. He personally told me he took all of his money out of the stock market and put it into Magic because the rate of return is infinitely better. See those Avacyn Restored Fat Packs for $35? Imagine what they'll sell for in 2016 and you'll pity yourself for not buying an extra 5 now... (see Steven's informative comment on the block sets generally being a better investment than core sets as well).

I know this isn't exactly what you were asking for, but if you want to make money buying and selling Magic products, actually opening them is probably one of the worst ways to do so IMHO. There's always the risk that prices will go down if the economy worsens or the game goes out of style, but that risk is inherent is virtually any investment nowadays.