Trying to identify the best method for IIR realizations
I don't think "regardless of application" works here. Every application has different requirements and hence the optimum choice would be different. 

However in gross oversimplification


Direct Form I is the "safest choice". Direct Form II is slightly better since it has only half the amount of state variables. Stay away from Direct Form II and Transposed Form I since the transfer functions to the state variables are only determined by the poles and there can be massive gain scaling issues
Cascaded is the easiest choice. It's fairly simple to convert between poles and zeros and actual filter coefficients. Parallel requires calculating the roots of a large polynomial which is a numerically ill defined problem. 
There are different ways to pair poles and zeros to create second order sections. Best one is this : http://scipy.github.io/devdocs/generated/scipy.signal.zpk2sos.html 
Use floating point whenever possible. Fixed point IIR requires very careful analysis and application specific implementation. 
Whether optimization is worth it really depends on the application. Requirements can be all over the place: precision, speed, memory, mips, ease of us, stability against outlier condition, etc.