In some ways, I don't have a signature Pokemon. In others, I probably have several.
Let's see... First, there's my original starter Pokemon, Turty the Torterra. He and the rest of his teammates are still on my Pearl cartridge because I'm conflicted about transferring them; I don't know if it would be the same... I'll be morally obligated to do it once DP remakes come out and Torterrite exists, though.
Anyway, because I can't really replace him I haven't used many other Torterra, and he's off in Sinnoh to boot. Not really a signature 'mon.
Lucario is a Pokemon that I used a whole lot by accident. My Pearl team - and most, if not all, Platinum teams - include Lucario, my Black 2 team includes Lucario, and my Y team includes Lucario. That's three Lucario in three generations in the first games of those generations in which it was made available. It was largely accidental, though, and I wouldn't really consider Lucario a trademark sort of thing.
Officially, Noivern is my current favorite Pokemon, with Sceptile equal to it or close behind (and with a rather confused jumble of other Pokemon behind
it). However, I haven't had an opportunity to use it competitively at all (thanks for 97 Special Attack, GameFreak) since it's kind of a niche pick, and the only perfectly-bred Noivern/Noibat I have were from other people. As for Sceptile, I have one from an Emerald playthrough that sits in the PC... so it has the distinction of being a favorite Pokemon that I effectively have none of. Neither of these is a signature 'mon.
So, yeah. That's my entirely too long post on why I'm apparently abnormal. Oh, I should say that Noivern is great support; I just haven't had a team it would fit in so far.