That is hard! I would have to say the iconic GMD SD40-2, SD60F and F59PH along with the American C40-8W and SD70 Standard Cab, The SD75i and SD60! Along with CN's SD40T-3, But since the F59PHs are what I grew up with (I grew up with GO Transit) and are very sentimental to me, I will choose them!
the GMD/EMD F59PH
built from 1988-1994
built for GO and Metrolink
GO's numbered 522 - 568
I do not know about Metrolink's numberings
GO units 565 - 568 sold to TRE in Texas
GO units 520 - 556 retired from 2009 - 2011, sold to numerous buyers
GO units 557 - 564 still in service today on GO!
My favorite units are 554 (today used as a part donor for the other sold F59PHs on a leaser, scrapped), 558 (In service) and 563 (In service)
The funny thing is that 558 and 563 are both on the same consist! on one of GO's L10L consists
GOT 554 at Oakville
GOT 558 on the Weston Sub, today this is a big trench, the iconic Strachan signal bridge is gone
GOT 558's current appearance, GOT 558 and 563 on the L10L on the line It runs on today, and in fact is the exact same consist!