HALX: the problem with the burnout serie is that it was good for 1 and then great for 2 and awesome for 3. 4 didn't bring much new to the party and then 5 or as they call it dominator actually took some features OUT!. so if you can find 3 or 4, then you should definitely give it a go! i haven't tried 5 because the fun part was the diversity of the styles of gameplay.
if you haven't tried the others, you're missing out on the crash mode. it was 50% of the game. Imagine playing pool with your car: you take off and try to hit cars or special items and cause the BIGGEST crash possible in terms of $. once you hit something, the crash starts and then you can't control the car except for the slowmotion mode where you can guide it just a bit. so the fun is really in finding how much dammage you can put for 1 intersection. Some of them are simple but others have 2 or 3 freeways and bridges over another so you have to crash, fall off and hit the next one, then use your nitro to explode again to fall on the other bridge. it's quite awesome.
I hope I gave you enough info. 3 has bonus multipliers in the crash mode but in 4, they give you a different style so theres no bonus but i find it more fun.
most people will say 3 is the best because they played it before 4, but if you want to try the crash modes, then go for 4. it will feel more fresh.
enjoy!