Easiest bicycle to assemble? Single speed?
I would suggest that before you decide to build a single speed/fixie consider if this is a type of bike you are likely to use when it's complete. One aspect that has to be considered is the cost of the specialty tools required for the assembly. While many enthusiasts consider the cost an investment, it is none the less an expense. Part of the problem with your first build may be determining what tools you will need prior to the actual assembly. Do you need a headset installation tool or will it be a drop-in unit. what bottom bracket type and what brand will require different tools. Will the wheels have serviceable bearings requiring special wrenches or pressed in bearings. None of this is insurmountable but can be frustrating. My suggestion is to take your time picking up components when the price and quality meet your expectations. Shop around noting the average price so you can take advantage of exceptional deals. Them acquire what ever tools you need to mount the specific component. I have done this several times and the time frames ranged from 30 days to 7 months.