Hi Tabmow,
If by powerful, you mean one that produces a LOT of vapor and has a good
throat hit, probably the best one is the Joye 510. It's a popular unit, and a number of venders sell it for 50 dollars or less. The cheapest seller I know, off hand,
Cignot your Midwest Electronic Cigarette store. has the standard kit, but without the box for $35 plus shipping.
There are higher voltage custom models, though, which don't look like cigarettes, which produce HUGE amounts of vapor. I'm not up on these, myself.