The formula that I use to decide upon my next champion.
This is a formula I use when buying new champions. This may not work for everyone but I've gotten quite a bit of mileage out of it.
First: I separate the champions of my choice into 2 primary categories based on what I am looking for in a champ and adjust priorities based on the formula, eg: Does champion use mana as a resource? If yes, move down, if no, bump up one level in priority. So here's the version I used when buying a 6300 champ:
*Does champion use mana as a resource?: if yes, move down, if no, bump up.
*Can champion play more than one lane?: If yes, bump up one slot, if no, move down two.
*Does champion have juking potential?: If yes, bump up two slots, if no, move down one.
*Does champion have high damage?: If high magic damage, bump up one, if high physical damage, bump up two. If neither, move down three.
*Does champion have good wave clearing?: If yes, bump up two, if no, move down three.
Using this formula, the most recent champion I purchased was: Zed.
Remember, this formula is malleable and can be adjusted to your needs.