As far as commercial vendors go I really preferred SQL Server from about version 2000 onwards. I got into Oracle again recently for a consulting project with a finance client (who basically have unlimited Oracle licenses) and really quite enjoyed it since the Oracle 7/8 days.
You can do some phenomenally sophisticated things... I extensively leveraged things like partitioning, parallel execution (dbms_parallel_execute!), lots of PL/SQL using the pipelined table cursor stuff, data mining stuff (dbms_frequent_itemset!), index-organized tables, and my god, bitmap indexes were a godsend, direct insert tricks for bulk data loading, external tables were fantastic (you can wrap a .csv in an external table and interact with it in parallel just like any other table -- great for ingesting large amounts of janky .csv data from other parts of the business).
The parallel execution and robust partitioning options were probably the most critical pieces that have no particularly good counterpart in open source land.
