1993 NH 2120 (the best), 1974 MF 135 (sold, but solid), 1947 Farmall A (bought, sold, bought back, sold again), 1956 MH50 lbt (sold, in 1980, darn it)
I'm not sure it matters much, as the water jacket is continuous around the cylinders. Most guys use the one that's easiest to get to to remove, and to install the block heater. The heater often takes a right angle inside, but should fit most places. Pick up an extra plug or two in case I'm wrong - they're cheap.
Jim