Dunenewt's suggestions

[Rippsblack]

obviously the first keys to come to mind would be T, I & H as rene suggested

but i prefer to think about the keys most comfortable to press

for example

Q

A

Z

are each at the left most side of the keyboard, and one per line

ZXC and would also be good canditates as they are all in same location

but in my honest opinion

Ctrl+R should be set to ®ecall all harvesters

Ctrl+F should be set to select all (F)oot soldiers

Ctrl+V Should be set to select all (V)ehicles

this should be easy to remember as they keys are again next to each other and on seperate lines :)