i'm trying to find the options contract that has the highest gearing/leverage (i.e. it will change the most % wise for each unit movement in the underlying) How do I find that one particular option contract? Do I just find the contract with the max delta/price? How about vega and other factors? Any site which list the "gearing" or "leverage" of each contract?

An option's delta describes the % the price of that option will change relative to a change in its underlying security. The highest possible delta is 1.0, which would be the delta of a deep in the money option. Vega describes something else - the sensitivity of the option's price with respect to a change in volatility.

The further you go OTM the greater the leverage. A ball-park formula is Leverage= Option Delta x (Spot / Option Price). That gives you the change in option value (%) for any given 1% change in the Spot.

It's not necessary true that the further you go OTM, the greater the leverage. I thought so too previously but practical experience prove otherwise. For eg. if I want to buy a SPY call option that have the highest leverage, which one should I buy? http://finance.yahoo.com/q/op?s=SPY

it's obvious from the question you don't know much about options and you shouldn't be trading this shit !

Both vega + delta are factors that describes the change of the option price. So should I factor in vega when trying to find the options with the highest gearing?

You're going to find it difficult to make money buying OTM calls on SPY. Each time the S&P500 goes up, the implied volatility of your long call will drop. Your expected profits - calculated using your option's delta - will vaporize like the illusion they were. That's probably why your "practical experience" has not matched your expectations. In other words, the vega is set up to work against you when you buy a SPY call. Of course it works both ways - the good thing is that if the S&P goes down, IV will go up and you will lose less on your call than you expected to.

It's not possible to answer that question as there are too many variables. You would know that if you understood how options work. Profittaker wrote an good answer above, that comes close for about one day, wiseguy. But go ahead and blow yourself up any day.