I have two signature Pokemon, one defensive and one offensive:
Defensive- Skarmory. HOLY STEEL-BANDED BIRDS, KEESEMAN! I mean, what is up with this? One of the best combinations in the entire Pokemon universe. Like, COME ON! A steel type impervious to ground type attacks, and fighting moves aren't super effective? And Rock type moves are also weakened? How amazing is that?! Best. Defense. Ever. I love you Skarmory.
Offensive- This one was much harder, but I'd definitely have to say Blaziken. It being my first Pokemon ever, I've always loved Blaziken. It's got some pretty kick*** moves, I've always loved its signature move, Blaze Kick (and hated when other Pokemon were able to use it), and it just looks pretty BA as well. Its defensive combinations are also pretty decent, seeing how it's a fire type that rock moves don't damage significantly. I would definitely say that this starter would be my first choice over any other.