I am a big fan of Xander, but I disagree. I'd definitely trade Xander for any of the Mets' big 3. However, if they do trade X, they have to keep Swihart at C, probably trade JBJ and bring in an OF bat that can hit 5th or 6th in the lineup to keep the offense viable. A bottom half of Pablo - Castillo - JBJ - Vaz - Marrero would be too weak.X is going to be a star SS for the next 15 years. If his power expands, he will be a super-start SS, face of the franchise player. He should be considered untouchable straight-up 1-on-1 for any Met pitcher.
Of the Mets' big 3, Syndergaard would be my top choice(although I can't fathom why the Mets would do it). He is 23 years old and under team control through 2021 (age 29); two years longer than Xander. He has an ideal pitchers frame being both tall (6'6") and sturdy (240 lbs) and features hard (~97 avg of FB and 85 avg on SL) swing and miss stuff (47.5%). In his first pro-season he put up an xFIP of 2.91 with 1.040 WHIP, 10 k/9 and 1.9 bb/9. Sky is the limit for this guy. Plus, that flow; Eck would be all over lettuce like that.
I'd probably be a little bit more hesitant with DeGrom and Harvey because of their age (27) and in Harvey's case his team control (one year fewer than X) and his injury history. However, I'd still pull the trigger. These guys are legit front of the rotation power arms. They have career xFiPs in the high 2.00s, average >9 k/9 and ~5 k/bb, their average FB velocity is ~95 mph and they get swinging strikes on about 12% of pitches. That's legit ace material right there.
That said, if the Sox want to keep Xander and take advantage of the Mets pitching surplus, I'd take a long look at Zack Wheeler. He's not quite as good, with an xFIP in the mid-3.00s and he gives up more hits and walks. However, he's still a guy who throws in the mid-90s with swing and miss stuff (~10% rate) and racks up Ks (> 9 k/9). Plus, he can probably be had for much less than Xander. He's probably more of a 2/3 type, but he essentially replaces Buchholz. They still need a top flight arm, but they would be improving the rotation for the future.