Well, you didn't say it before, so I mentioned it. MS is good a) on Luke b) against Dark Curse. That seems enough use for it to not be the worst upgrade.Confirmation Bias
a) Lots of upgrades are good on Luke
b) Dark Curse isn't a threat
All the other upgrades (except for Expose) are cheaper and/or do more. The upgrades that only cost 1 aren't great but they are good for reaching the point cap to avoid initiative. The upgrades that cost 2 are all better most of the time or have important uses. Push the Limit is definitely better and Daredevil is better on ships with a boost icon (terrible on ships without it, though). Expose is terrible most of the time and costs more so it probably will be the worst when it arrives.
If I had to choose any upgrade card on an X-Wing, I'd choose Push the limit. It turns any ship into Vader and the stress token will be irrelevant much of the time because it will be possible to choose a green maneuver before the token prevents another action from happening. Also, PtL is a better damage increase on X-Wings (or any ship with 3 attack dice) because it allows Target Lock and Focus in a single turn on at least 3 dice which is usually better than +0.3 average damage a turn against Imperial ships. Pushing through an extra crit is worth less than +0.25 extra damage when not taking into account the other effects.Marksmanship
and Expose are the worst because everything else is better.