Small engine - ianhad
As sub-1500cc engines rate a lower car tax, what is the most powerfull car on the market with such an engine?
Small engine - Andy P
After consulting my bible, I came up with the following:

1500cc

Toyota Yaris T Sport 105bhp
Hyndai Accent 1.5MVi 100bhp

1400cc

MG ZR 105 105bhp
Honda Civic 90bhp

However, if want the ultimate perfomance, then try the Caterham 1.1 Blackbird Roadster....with 170bhp (0 to 60 in 3.9 secs and a top speed of 139mph)

Personally, I'd go for the Toyota

Andy
Small engine - IanT
To be absolutely precise on tax rates ...

Cars first registered before 1/March/2001:
Less than 1550cc, tax = £105 per year.
1550cc or above, tax = £160 per year.

Cars first registered on or after 1/March/2001:
Tax is based on CO2 emissions, which are listed for most new cars on www.vca.gov.uk
Cheapest rate is petrol engines with CO2 <= 120g/km, at £70 per year.
Next best is diesel with CO2 <= 120g/km, at £80 per year.

As Andy P says, the best performance from a small engine will come from a specialist sports car manufacturer - a lightweight body with a tuned engine. But most of these aren't listed on the vca CO2 database, so it's anyone's guess what their tax will be on the new rates.

Ian
Small engine - TrevorP
OR plan B -

Seat Ibiza Sport PD130 - £110 p.a.
Small engine - Ben79
Why is anybody worried about road tax cost? The difference between the highest and lowest cost is just a small fraction of the running costs for a car. £50 would be taken up by a small difference in fuel economy, insurance group differences and dealer charges.
Small engine - jc
Don't just go by the power;divide this by the weight.This will give power/weight ratio-the higher this is,the better the performance!