In this order:
Halo
Mass Effect
Borderlands
Halo: I fell in love with this series when I first played it at a kiosk in the South County Mall's GameStop right around the launch of the Xbox. It was the reason I bought an Xbox along with Dead or Alive 3 and Oddworld Munch's Oddysee. I nearly creamed my jeans (figuratively) at the E3 announcement for Halo 2 with it's jaw-dropping graphics. I bought an Xbox 360 for Halo 3 and Halo the Master Chief Collection and Halo 5 Guardians were the first two games I bought for the Xbox One when my wife won the system. It has consistently been a series at the top of my must-play list, to the detriment of finishing other games. First, because I just kept going back and replaying some of my favorite sections (the Silent Cartographer in Combat Evolved is a perennial favorite of mine), then when I finally got high speed internet where I live in the boonies after Reach launched, I just kept playing multiplayer matches because it felt. Just. Right.
Mass Effect: One of the greatest sci-fi epics of all time, across any format whether games, music, movies, books or whatever. This series truly made me care about the characters in my team and I genuinely felt a sense of loss when somebody died or even just left because of differences. I don't usually take to internet protesting and in fact, have only done so twice. Once was on Major Nelson's blog to protest the boneheaded decisions Don Mattrick made with the Xbox One (hence why I refused to buy one and only own it because my wife won it) and the other was to join the "Retake Mass Effect" movement because the developers took a big shit all over their fan's faces with the crappy way they ended it That's how much I love this series.
Borderlands: I have a love/hate relationship with this series. I love the art direction. I love the humor. I love the plethora of weapons. But after the first game, the second just never clicked with me the same way. And I hate Randy Pitchford - the ass in charge who fucked us over with Aliens Colonial Marines.