A note on the illegality of guerrilla gardening
By now, you'll have no doubt realized that guerrilla gardening is perhaps a little on the illegal side. People, companies or local government will undoubtedly own the land which you are going to cultivate, and interfering with other people's land is illegal. Unless you can obtain permission to garden on the land in question, you should abide by three rules: Use only land that is unused or unwanted.
The land that you pick should be unused now, and for the duration of your vegetation's lifespan. Your definition of "unused" is up to you; derelict land is unused, but what about the grass verges on the sides of roads?
Leave the land in better condition than when you found it
If you're going to use some land for guerrilla gardening, then you should leave it in a better state than when you first took an interest in it. Improve the fertility of the land with compost, go organic, and clean away the detritus of urban life. [...]
What to grow in your guerrilla garden
Chances are, with a guerrilla garden, you'll be working with a small piece of land in a largely urban area. You'll be growing in an area that the general public will not regard as a garden or farming area, so you should expect your garden to be ignored, or worse, trampled over. So, you'll need to use plants that are hardy, low maintenance and that have a high success rate if you want anyone to sit up and take notice.
If you want to extract maximum usefulness from your reclaimed garden, then I would recommend growing some form of vegetable or fruit and growing your own food. If it's shock value you're after, then a spray of bright flowers raised elsewhere from seedlings and transplanted out into the wild once established should achieve the desired effect.
Guerrilla gardening has, of course, many limitations and can, when it moves too far away from community gardening, become more activism than gardening. A better alternative in the long term would be a real implementation of urban agriculture, like Cuba did when they stopped getting soviet oil. I plan on writing about the subject in the near future. Stay tuned.