I don't really have an opinion about the Gen VI starters yet, since we only know about first forms right now, so I just limited my choice to the first five gens.
For me, the top was actually Gen V. All the starters at all their evolutionary levels were highly appealing to me, which is unique in terms of starters.
In second place is Gen I. All of the starters and all of their evolutionary levels, except for Charmeleon, were highly appealing. Charmeleon was just ugly to me, though, so it wasn't quite as good as Gen V.
In third is Gen IV. Loved them all except for Chimchar's line, but even that is growing on me. Infernape is pretty cool.
Fourth is Gen III. Now, Gen III does have one of my all-time favourite starters in Blaziken, and I love Swampert too, but I really disliked the mid-evolutions of both Blaziken and Swampert, and I find Treecko to be pretty ugly.
Fifth is Gen II. Meganium is cool, Feraligatr is okay, and I'm growing more fond of Typhlosion, but all in all it was pretty disappointing in terms of starters, to me.