For me Design is one of the most important aspects when choosing a Pokemon. My top 5 would be: (in no particular order)
1. Blaziken. I think a lot of thought went into Blaziken. It was originally going to be fused with Latias for one Pokemon, but GF made them 2 separate Pokemon.
2. Blastoise. Starters in general usually get the most time I think when it comes to design and Blastoise was built to Perfection. One of the best gen. 1 Pokes out there.
3. Starmie. Starmie is a VERY unique Pokemon with its moveset and all, but its design is so simple it is magnificent. It is just 2 stars stacked. I love Starmie.
4. Jellicent. There is a reason I am a Jellicentfan. I love its design. You know that GF thinks a lot of a Pokemon when it makes 2 forms of a POkemon. Jellicent is special to me. Its design is so unique and really great.
5.Reshiram. Reshiram is one of the coolest looking Pokemon ever. I got stuck with the White version so I missed out on it, but it still looks amazing!
5b. Castform. I can't forget my favorite pokemon of all-time. I like all of its forms. Normal form is still the best though.
Honorable mentions: Breloom, the Latis, The legendary birds, Volcarona, Butterfree, Scolipede, many others.