Well, asking what the best nitro engine you can buy is like asking what the best car is, some people may say the hennessey venom GT, because it's the fastest, but some people will say a Bentley continental GT because it may not be as fast, but its more comfortable, anyway, I'll shut about cars and say as a spare just have the stock engine for your car. If you want another engine with similar performance to the B5, loom towards the B6 (smoother bottom end, better for buggys) or the Novaross P5XLT, or any engine from LRP, O.S or GO engine in your price range that's got good reviews. It depends on the person.