For me, the series that I've played religiously was Halo. I played the hell out of the first one and beat all three of the campaigns on Legendary. Also I played and beat all of the Call of Duty games (minus the shitty ones) on Veteran.
I absolutely HATE meaningless challenges/time trials in any game...it's hard for me being such an achievement whore. I hate saying "Ok...I'll give this challenge/time trial a try" and then ending up in the fetal position, having hit the retry button 30 billion times and still a point or a second off from getting that fucking gold metal!