The three variants you're going to see by far the most of ar the No. 1 Mk III, the No. 4 Mk 1 and the No. 5 Mk 1.
The No. 1 dates through WW1 to the beginning of WW2 (later in places like Australia that didn't switch to No. 4s.
The No. 4 started in early WW2 and continued through the war.
The No. 5 is a carbine variant made in relatively small numbers.
The No. 1 looks the best (you can disagree, but that just means you're wrong) and it has the blunt nosecap. The No. 4 looks more "normal" with the barrel protruding and the carbine usually has a flash hider.