Which Pontiac Firebird year(s) are/is better?

2000+, the LS1 (code name for V8 engine in the late model year F-body) is a bullet proof engine in your years stated. You can modify it easy, it will last a LONG time if you perform normal maintenance, well over 150/200,000 miles. The build quality on 4th generation Firebird's is higher than the older 3rd gens.