The Mario Kart series which I would never include in a personal favorite list unless it's a top X personal favorite racing games list. I love the style, the controls, the characters and even a lot of the tracks. It's a really colorful, imaginative series that you can just pick-up and play at any time really. That being said though, there are a couple of things that keep me from loving the game.
The petty gripe, more than likely a product of modern games, is the lack of unlockables. Most Mario Kart games I've played gave you every character and track (excluding the special cup) which is great for a pick-up-and-play game but makes single player seem pointless if you say, have an unfinished Dragon Quest or, Final Fantasy or, Zelda game on hand. Later games bring in new characters though which is great for single-players like me.
The biggest issue is how easy it is to get your ass handed to you during a race. It almost seems like your opponents get items at random and the lack of a shield power-up really makes you feel vulnerable. In Mario Kart 7 I've been ahead of the pack for a lap and a half only to get blindsided by a blue-shell, lightning and, whatever I had trailing behind me as a means of self-defense.
I love the games but have never been in love with the series.