I'd say so far n-starts. When I started they went for 420-440, now they go at about 520. That's about a 20% increase in 2 months.
A star tool will give you the highest reward although it's also the most expensive. If you've got everything lined up for you (perma to a farm town, an untouched mine) then you'll be able to make your money back and then some fairly quickly.
The problem is they can also go down, they used to be 500 like 4 months ago and then they dropped to 400. I guess that's the risk with any investment!
Yep they can, but the risk-reward ratio seems pretty good with them. I will keep an eye on them to try and notice a trend. Other than n-stars, the star tools and SG kits are no doubt the best investment. But with these you actually have to work, while n-stars, when the tide is right, do the work for you. It's the simple 'money cmakes money' principal.
According to what I have been reading in other forums - kits are probably by far the best investment at this time - players are stating they are making thousands of dollars a day. I do not speak from experience, only from word of mouth. That said, I would be careful as I believe the rewards will be lowered if the economy of ECC is to survive. Remember, you are playing minecraft, so mining should always be the best economy IMHO.