Can I use your time machine with radio recording equipment? There are very few broadcasts known to exist pre-1932 (and not that many from the 1932-1935 period), and I’d like to tape record pretty much everything.
Sticking to television (and, to start this, American television), I’m tempted to say “record everything”, but, if that’s not possible, some early selections include:
The 1940s and earlier. Virtually nothing survives pre-1948, and there isn’t much surviving from 1948 and 1949. High on the list here: The first day of the NBC network in 1941, the World Fair broadcasts of 1939, experimental programming of the 1930s, and the 1948 election coverage (conventions, election day, and everything else) beyond what currently exists.
Newscasts. Most newscasts pre-1968 are gone (and a lot of those that survive between 1968 and the late 1970s survive only due to the folks at Vanderbilt University). I’d like to record some of them, especially from ABC, which seems to have a survival rate even worst than the other two networks.
Local programing. This tends to have a bad survival rate, especially outside the major TV production centers.
Color videotaped programming. Very few color videotapes exist before the mid-1960s, and I’d like to have some more of those.
Sports. Sports coverage (and especially coverage of baseball, basketball, hockey, and football) into the mid-1970s has a survival rate worst than that of virtually all other kinds of programming. In particular, videotaping some color World Series broadcasts in the late 1950s (the earliest to survive on color videotape is Game 3 of the 1969 WS).