All else equal a CRJ900 and E175 have a higher ratio of first class seats to economy seats than other aircraft types, so routings offering those aircraft might be a good bet, but of course those planes (or at least the CRJ900) are arguably less comfortable than other aircraft types. And by contrast, the 3-row first class cabin A320s offer a particularly bad ratio, so avoid those if possible.

You could also book whatever is cheapest now and then try to do a same-day change the day of/day before to whichever flight seems to have the most first class seats open. But unfortunately, GM upgrades are much rarer than they used to be, and even if you SDC to a flight with a lot of first class seats open, you may find there are still lots of DMs and PMs ahead of you waiting on the upgrade list. So maybe just pick whichever flight gives you the most comfortable economy seat option (notably E170s/E175s and overhauled A319s/A320s)?