My second love is Traveller. To the point where I own a few books of every one of its incarnations.
Still, plenty of other game systems I would play, or run, in a heart beat. Of course, technically, D&D is no longer my first love. My C&C version of it is. Pure versions of D&D are my "1.5 Love".
In a attempt to rank the rest of what I "love":
L5R, especially the newest 3rd version of it. I love how the mechanics and the flavor of the setting complement each other, and is deadly, but not too much so.
Chivalry and Sorcery. I just fell in love with the rich detailed flavor of the whole thing.
Paladium Fantasy. I am not so much in love with the system, but the world is definitely one of my favorites and I ALWAYS draw upon it for campaign and adventure ideas.
Shadowrun. If you have played this you already know why. Just too frickin cool.
Mutants and Masterminds. It finally did supers in a way that I finally played and enjoyed the genre.
RIFTS and Synnibar. With some tweaks they are super powered fantasy done right. Just a lot of fun.
EPICrpg. I haven't actually played/ran this fully, but I keep coming back to it because, like L5R, its mechanics and flavor seem to combine very elegantly. Maybe I should say richly. Anyways, I need to make time to run this fully some time, and find out for sure.
There are others I just "like", but those listed I think I can honestly say say I love to some degree.