Have you checked your Reference Manual (installed with a full installation in Program Files (or x86 if you're using 32 Bit) /QlikView/Documentation/
Check out from Page 369 (Aggregation Functions) until page 469 (includes Other Functions) - Note: Page Numbers may vary with different QV Versions, but check out the index anyway.
Basically the types and frequency of functions you use I guess would depend on your industry (Research using more statistics, sales using more point in time analysis (using Set Analysis, ofcourse!), etc.
Hope this helps.
Len() to check for blank/null since QV is a bit weird about null. I usually stear clear of IsNull().
Match() for saving space in comparison logic.
Applymap() for mapping.
Most of the others would just be random uses of aggregation and string manipulation functions. If you are in a statistics or finance industry you might use some of the math functions a bit more often.
Use a lot of Set Analysis in expressions.