Paul,

If it's 6061-T6 you don't need anything better, it will hold threads and its as tough as you'll ever need it to be. There is 7075 (not sure of the tempers available), but that's some pretty expensive stuff, overkill.

For general use 6061 is OK.  The T + a number indicates the temper.  "0" showing no temper.  For ultimate strength and machinablity 2024-T3 and 7075-T6 are much better, but as noted more expensive and harder to find.  Also I have found some 2011 extruded shapes at McMaster-Carr, round and hex, that are an absolute joy to machine for spinner nuts, venturis, etc.  A machinist friend of mine used to refer to 6061 as "GUM"!