Functions you need to know and use that most people don't

• VLookup: A no-brainer.
• Index & Match: As in Index(data, match(), 1,0).
• Rank
• Conditional Format Matching
• Named ranges

You can get a long way with these 3 more advanced functions. When you really need the heavy guns, pull out

• Sumif arrays
  
• Offset
• Indirect
• Data->Table for scenario modeling/sensitivity