The Amazing 300 (600) F4.
A lens that has really empowered me to take on pretty much anything long lens related, is the Olympus 300 F4, which acts in the MFT format as a 600mm for magnification, but has the effectively depth of field of an F4 300.
Over half way across the field.
With plenty of cropping available.
And when you are waiting for the action to return from the far side of the ground, this is at your feet.
With again, a ton of cropping ability. Only my 75mm is its equal, sharing that quite rare smooth-razor sharp look.
Weatherproofing, a carry all day form factor that fits into a regular bag, awesome AF, semi macro close focus, dual stabilising, sharp wide open edge to edge and 70% cheaper than full frame equivalents. What is not to like?