And by the way, Lucario is still popular. He continues to get some promotion, be it the Lucario figurine that come with Pokémon Rumble or a man in a Lucario Suit appearing on Pokémon Sunday.
I would like to stress this. There's these figures getting sold (in gachapon machines) in Japan that can be used Skylanders-style in Pokémon Rumble U - you put this figure on your GamePad and it'll appear in the game. There's one set released at this moment, a second coming may 25th. Each set exists of six figures and a special seventh. The first set is composed of Bulbasaur, Pikachu, Torchic, Piplup, Victini and Lucario (and a shiny Pikachu as the special seventh). These Pokémon seem to be very popular: the first Pokémon; the face of Pokémon; two starters that gained lots of popularity via the anime, where they were used by Ash' female companions; the latest Mew-alike; and Lucario.
The second set seems to be composed of Pokémon that are quite popular but not as much as the first four: Eevee, Mewtwo, Croagunk, Zoroark, Litwick and Genesect. In case you're wondering why Croagunk is on there: he is used as comic relief, being the Pokémon that drags Brock away from the 'pretty ladies'. I have no idea why Litwick, though.
Simply stating that Lucario is more popular because he appears in a set before Zoroark and Genesect is too easy, but there's more. I remember fondly how, when I went to see the first Pokémon movie in the cinema, I got this Mew promo card. They're pretty much doing the same thing with these figures, releasing promo figures. At this moment, there's two known: Pikachu, of course - and Lucario.
Then there's also him appearing in Pokémon Sunday, as FalKoopa said.
That being said (and my apologies for the semi-wall of text), I don't think Lucario is going anywhere - especially since there was quite the uproar when Mewtwo was cut from the roster.
My guess (and I'm feeling quite certain about it):
1. Pikachu
2. Jigglypuff
3. Pokémon Trainer
4. Lucario
5. Mewtwo (with his Awakened forme as his Final Smash)
My backups would be, in this order:
6. Genesect
7. Generation 6 Pokémon
8. Zoroark