Playing Starcraft 2

The machine with the dedicated GPU (i.e. the iMac) will *smoke* the other two machines, which do not have discrete graphics. I have a 2014 13" Retina with "Iris" graphics... the performance is rather poor compared to my 2012 Retina 15", which has a discrete nVidia 650 GPU.

Integrated GPUs are improving, but they cannot hold a candle to discrete GPUs - even ones that are several years old..

You asked which machine would perform the best. That is the question I answered.

I've played SC2 on a non-retina 13" MBP with HD4000 graphics. It ran fine with mostly-medium and some low graphics settings. The SC2 preferences tell you which options tax the CPU, and which ones tax the GPU.

I can't say exactly how things will run for you because playing Starcraft at Retina resolutions with an integrated GPU will result in lower performance than my tests at non-retina, 13" resolution (i.e. 1280x800).

If you are *really* asking which of your machines will play SC2 in an enjoyable manner, then they all will, although it's a matter of personal preference.

You're going to get some serious fan noise and heat on all three of your machines. The iMac will probably "handle" it the best, but your other computers will be fine, too. I used to have an old 13" MBP that essentially did nothing but render from 2008 and it only recently started having problems.

SSD won't give you much, if any, of a performance boost for SC2. As far as "taxing" the machine goes... dude, don't worry about it! Folks who get on and play around in Aperture tax their CPUs/GPUs just as much!

You can play SC2 every day all day without worry. The reason your computers have that much horsepower is so it can be leveraged! If your fears about needing to "baby" your machines (which I totally understand) prevent you from using your nicest Mac to play games, then go ahead and play on the weaker Macs.