Not in any order:
Zelda: Should be obvious.
Metroid: An excellent series thus far, the transition from 2D to 3D was practically seamless, and every important aspect was preserved pretty well. The next installment, Other M, looks like it shouldn't disappoint, and I can't wait for another main series update (main series referring to Zelda, Star Fox, Metroid, etc.)
Call of Duty: I've always had a passion for the games in this series, especially the entries by Infinity Ward, though, Modern Warfare 2 had some flaws and features I wasn't too happy about.
Metal Gear Solid: An entrancing intricate stroyline, amazing plot twists, and awesome combat. What's not to love?
Mario: Should be self explanatory.