In my limited experience, HEC1 does not calculate 100-year elevations, but merely 100-year flows.  It does this based on SCS curve numbers and stream channel characteristics.  Depending on the size of your stream's drainage basin, this could be incredibly cumbersome, or just cumbersome.  For smaller drainage basins, I use the SCS Curve Number method and HydraFlow to calculate my 100-year flow, while for larger drainage basins (measured in square miles, not acres), I use USGS Regression Equations.

I have no experience with HEC2, other than everyone saying: "So you escaped that nightmare, aren't you lucky?" HEC-RAS is the software you'll want to use to determine the 100-year elevation.

HEC1 is grotesquely outdated. Its input and output is text-based, with data fields refered to as "cards", and my first experience with HEC1 output left me with a 64,000-page output file. I would recommend that you give it a big swerve.

I recommend that you look at FEMA guidelines for their studies as a basis for your methodology.  

Santosha
HEC-1 is a hydrologic model whereas Hec-2 is a hydraulic model, as francesca stated. Both are old dos versions.  Now we have available to us both HECHMS & HECRAS, hydrologic and hydraulic respectively.  Other methods are available for computing discharges.  I would suggest that you contact your regulating entity (city, county or state authority) to see what they recommend. For example in  my area we use the rational method for watersheds less than 160 acres and HEC-1/HMS for anything larger.

If your drainage basin is fairly large, say 100 acres, I'd suggest the use of Regression Equations.  This is what many flood studies use if no actual streamflow data is available.  You can use the National Flood Frequency Program (NFF) available FREE or simply use the equations published for your area by USGS in the United States.

HEC-1 is old and cumbersom.  HEC-HMS is newer but requires a lot of input.

HEC-II and HEC-RAS do NOT calculate flows.  They calculate water surface profiles for the flows you provide from some other source or method.

If you are not in the US I don't know where to direct you.

good luck

Thanks Guys, your suggestions were incredibly helpful.  Much appreciated.

HEC-RAS is an excellent program to use to calculate 100 yr flood once you determine your Q using either HEC-HMS or other type of software. If using HEC-HMS and using the SCS method, you will only be required to enter 1 parameter (CN #). HMS seems complicated, but if you complete a training course, it will help u alot.

Also depending on drainage area size and the location you are in! What I mean by location, is the results of using the regression equations are very unrealistic in my area for the 100 yr Q. Therefore, use more than one method to double check you Q's.