Well, you have to first consider a budget. then find the best flash to fit that budget.
If you're on a tight budget, you may consider the Canon 270ex. Its small, light, and the head can be turned to point upwards, i.e. bounce flash.
If you're more serious about photography, then invest in the Canon 430ex series, which is a mid-range flash which should satisfy most users. it has faster recycle time than 270ex, more powerful, yada yada.. basically its better than 270ex. but bigger and heavier.
then if you feel like splurging, get the top-of-the-line 580ex II.
It's your choice..
but if you ask me the 430ex is a gd flash to begin with.