My Top 12 favourite fully evolved Sinnoh Pokemon:
I actually don't know which starter I like more, Infernape or Empoleon. I have fond memories of using both of them, though Infernape was usually my go-to choice simply because there were so few fire types (especially in Diamond and Pearl!)
Staraptor always gets on my team whenever I play a Sinnoh game. The Brave Bird/Close Combat mix puts in too much work not to have it!
Luxray was the lion before Pyroar came along and mixing Thunder, Fire and Ice Fang with Night Slash leaves you with a monster! Just wish someone would give it an electric/dark mega!
Gliscor is one of those pokemon who I took one look at and thought "Well, they were bound to start coming with abysmal designs sooner or later" - then Ash caught one, so I trained one and suddenly I was all turned around on the idea. This is currently the only occurrence of the anime making me like a pokemon which I initially hated the look of (it then went on to do the exact opposite with Oshawott, but that's a different matter!)
Floatzel I just like. This thing was my go-to water type whenever I chose Chimchar over Piplup and it's always fun to use, even if it's not all that competitively viable!
Togekiss is the gift from above that the Togepi line needed. After seeing Togepi in the anime for so long before its in-game release that when we finally got a Togepi for ourselves I found it to be such a letdown, and Togetic didn't help at all! Thankfully we were eventually given Togekiss put put everything right!
Lucario and Gallade are always great to use, I've loved Aipom since I first saw it, so Ambipom was always going to win me over even if I didn't like its design at first, while Garchomp is a draconic land shark. What's not to love there?
I didn't really go much on Lopunny for a long time. I never seemed to be able to get Buneary to evolve soon enough to learn return and to me Lopunny was just this odd normal type that I never really got to use.
Then I did a wonderlocke on Pokemon Y and a Lopunny ended up on my team. By the end it was a staple of the team, one of the core four that carried me through to the end (along with a shiny Espeon, Pancake the Goodra and Mapleleaf Serperior - sadly, Mamuska the Darmanitan didn't make it to the end)