I'd guess either 73 or 320, but it changes, suddenly and radically.

For years it was the 717. No contest, not even close. Then the 73 and 320 took turns. For a little while the "EWR mafia" would ferociously write their reps demanding EWR trips, and suddenly they would multiply in the bid pack. The vast majority of the base despises them, so would call their reps saying "****?" and reps would say "we hear you loud and clear, 100% of NYC pilots lighting up our phones demand EWR trips!" then the silent majority would speak up and they'd be drastically reduced. This silly cycle went on for a while but now it seems to have stabilized closer to the minimum than the maximum most months.

Marketing is in charge of the airline now and they won't hesitate to make massive, sudden shifts with no warning. If one fleet can make one penny more for the next 1-12 months then they'll switch the current fleet's trip originations very abruptly. While that's probably more than you were asking, the point stands that no fleet has or ever will have reliably large amounts of EWR trips. What little there is will usually be snatched up by senior locals more often than not. So it all depends on how much better EWR is for you than JFK/LGA. If EWR is a dream drive and JFK/EWR are nightmares, seriously consider commuting to whatever base is easiest to get to from PHL. If you could drive to all 3 without too much hassle (at all hours, all traffic, all weather) then obviously bid NYC ASAP and hope for the best each month for EWR.

One variable is a possible BOS base if that commute would be easier than other current bases. It was pre-announced in 2019 for 2020 but then you know what happened so it got back burner'd but IMHO will be back next year sometime. Most likely it will be the 320 based on past and current trends.

Anyway good luck!