If you are travelling from the East or South East, then Melbourne Central is generally the better option (unless you are on a direct train, then it would be better to change trains at Richmond and again to a tram at Melbourne Central or get the train from Flinders).
If you are coming from the west, then North Melbourne -> 401 bus is the best option.
Also, depending which side of the uni you are going to, different trams may be better. E.g. If you are going to the west side (e.g. Med buildings, etc) then get the 19 from Elizabeth Street. If you are going to the east side, then Swanston street is the best option.
They will be making trams quicker along Swanston street soon though.