Eyeshield 21 is one of the best shonen manga running. It's funny in a fresh way. I love when serious moments are downplayed with completely offbeat comedic timing. And you gotta love Hiruma, the team quarterback. He first comes off as violent, blackmailing, gun-happy and, well, just batshit insane. And while some things never change, you soon perceive other surprising aspects to his character: he's brilliant, dedicated, a great strategist and an amazing leader.
Looking for romance in ES21? What you will find is a fascinating relationship between Hiruma and Mamori (team manager): threats with brooms and guns, a secret hand language, support with a side of teasing, and mutual understanding. It's fun and crazy and unexpectedly sweet. Also, Sena (running back) and Suzuna (head cheerleader) have a great friendship with hints of something more.
There's something to love about every team member of the DevilBats, as well as the members of other teams. Note, there is no "bad" or "villainous" character in ES21. Rather, you'll find (mostly) multifaceted rivals who you don't want to see lose either. Speaking of losing, someone below me mentioned how he/she thought it was boring how the DevilBats win every game. If I'm not mistaken, they lost a couple and tied once...Each match is really unique (play-wise) and I always anticipate Hiruma's secret moves and the growth of individual DevilBat players.
Other things you'll love about this manga: beautifully detailed art, the themes of teams (complete with cosplaying cheerleaders), football made simple though not unrealistic, Hiruma's crazy stunts (for example: confiscating everyone's passports and practically kidnapping the team) and blackmails, hilarious omakes/"investigations," gambling in Las Vegas, underage drinking (lol!) and much more.