I'd say my top five favorite Pokemon introduced in Generation 5 would have to be:
Oshawott/Dewott/Samurott - I fell in love with Oshawott from the moment it was revealed mainly because I find otters adorable. I don't mind the fact Samurott becomes quadrupedal, it's a great design and will always be my favorite starter in Unova.
Rufflet/Braviary - What can I say? He starts out as a cute little eaglet and evolves into a powerful bird. My only criticism in BW is how you have to wait till Opelucid City to capture one. At least in B2W2 you can capture one earlier in the desert area.
Deerling/Sawsbuck - I love Sawsbuck because of multiple reasons, artistically it being grass type makes more useful than Stantler. Ascetically the fact it changes appearance every season gives it personality. Winter has to be my favorite form!
Sandile/Krokorok/Krookodile - I feel in love with with this whole family mainly because of how powerful it proved to be in my first playthrough of BW. Krookodile has lots of moves to choose from and can cover lots of weaknesses. The fact Ash caught one in the anime made me fall in love with it even more.
Zekrom - Zekrom was the first legendary Pokemon that I actually felt a strong connection too. It reawakens at the perfect moment and chooses you as it's trainer to help save Unova. Everything about it is perfect, it's sleek black design, it's unique type combination of Electric/Dragon, and the fact in embodies the concept of ideals.
I love this spooky chandelier, it easily makes my top 10 Pokemon of all time. I adore the colour scheme and design (the blue/purple flames and yellow eyes just look great), it has two great abilities and a solid moveset (gets Ghost, Fire, Grass, Psychic and Dark moves, it's awesome) and I even love Litwick and Lampent! Ghost/Fire is a superb typing and I just like using it? Next time I get through Unova I'm adding this thing to my team, may even get an egg for the start of the game because it's only weakness is how late you get it in BW/BW2. Oh and it has a really cool cry.
Not the original Bug/Steel type, but goddamnit do I love Escavalier anyway! It's evolution method is pretty wacky (annoying for sure, but still kind-of cool from a lore standpoint) but the design is superb and it has a solid moveset all things considered. I also love its cry, for what it's worth. Kind of wish it was seen more in the anime (Ash could've used one, maybe giving Cilan Accelgor or something) but still it's one of my favourites.
Eelektross was one of those Pokemon I always had to use whenever I made a Unova team. Not sure why - could be my love of Electric types, how it had no weakness thanks to Levitate, the colour scheme, the wide movepool or the fact that it was just something rare that my friends never had the patience to hunt down and catch. The Tynamo stage always sucked despite grabbing it before the Flying gym, and Eelektrik kind of felt redundant since you probably got the Thunderstone before you found a Tynamo anyway? But still, I love the eel.