They all sound delicious! I would personally vote for something chocolate based (I know red velvet has cocoa, but it isn't overtly chocolate). That said, lemon is one of my favorite flavorings, but I have not yet found a lemon cake recipe that is as lemony as I would wish - if you find one, please share!
I'm eying the Joy of Baking's Lemon Cake recipe - it seems to be a pound cake recipe with lemon zest and fresh lemon juice in the cake batter. If I did that, I might make a lemon-mascarpone frosting to mix in, and use the 'lemon glaze' the recipe calls for as the dipping shell, instead of white chocolate.
Yay new experiments!
This gets my vote.
I looove lemon pound cake. Instead of the mascarpone, I would stick with the lemon glaze. Just because the pumpkin and other choices are heavy in flavor . . . a simple glaze makes this a lighter choice. Does that make sense?
I'm not a huge fan of frosting. So I may be biased. 
Oh, you may have misunderstood. The way you make cake balls is to basically shred the warm, just out of the oven cake into little pieces, and mix in a moist frosting, so that you then have the consistency to mush it into the balls. The glaze wouldn't work for that part of the process, because the consistency isn't right, and because it would make the whole cake ball entirely too sweet for anyone to deal with.
I thought of Lemon-mascarpone frosting because that would give you the moistness you need, without the heavy and and over sugared hit of a buttercream, and without the particular tang of the cream cheese frosting that I use for the other two. Lemon-mascarpone would allow me to, hopefully, keep it lighter and airier and retain the lemony goodness, while still getting the consistency right.
The glaze, though, might just be perfect for the outer shell of the cake ball, instead of dipping them in the traditional chocolate or white chocolate. Just can't let the glaze be too thick though, or the sweet will probably become overpowering...
