Umm, I don't know if I could choose just one but I'll list my top four and give explanations.
1.Nightcrawler:Quick, agile, and can get in and out of places without being noticed.
2.Iceman😛hysically strong but with ice powers for backup, plus the fact he can practically fly by making a bridge of ice in the air for him to slide on.
3.Colussus😛retty much invulnrable when he turns to steel, super strong he would deff. prevail in almost any fight he gets in, except for verse Magneto.
4.Wolverine😮ne word, Regeneration
If you were to combine those four x-men you would be unstoppable, nobody could really hurt you due to colussus' powers, what hurt you got, would be healed so quick due to wolverine's powers, you wouldn't be so slow when in metal form due to nightcrawler's powers, and with ice man, slowing your enemies down and stopping them in your tracks while you power up a frozen clawed super punch using Iceman, colussus's and wolverines powers. Hello, can you say super team,