SQL Server 2000 On Win 2000 Professional

SQL 2000 Personal Edition. The various flavors and requirements of SQL are detailed here:

http://www.microsoft.com/sql/evaluation/sysreqs/2000/default.asp

I would recommend Microsoft SQL Server 2000 Desktop Engine (MSDE 2000):

http://www.microsoft.com/sql/msde/downloads/download.asp

Purely depends on your requirements, if you just want to use it personally for experimenting or product development then SQL Server 2000 Personal Edition2 or SQL Server 2000 Desktop Engine (MSDE)are the best
however if you need a proper Enterprise solution i.e. if your requirements are beyond single site and involves extensive operations like replication, administration and OLAP etc. then SQL Server 2000 Enterprise Edition and Standard Edition is the preferred solution