Probably be United, but I bet likes of City and Liverpool not far off because of their more recent success. Real or Barca probably the biggest team stateside and globally since the Messi v Ronaldo games of 2010's.I used to root for The Pack when I lived in the Midwest. Now, folks yelling Come on you Yankees! Or Dodgers (both baseball teams here) Or for Dallas Cowboys, a football team that used to give my 49ers a tough time. etc.
I think all Premier League teams have followers in the States. Man U with the most, dunno who's last.
49'rs v Cowboys NFC final games used to be really the super bowl game in early to mid 1990's. Think free agency and salary caps stopped them being the leagues two juggernauts. 49'rs come close this year, four years ago and in 2013 Superbowl against Ravens.