The cigar is my favorite kit, for just pure satisfaction on a standard build. When the pen is finished, you have something that feels really solid and tough. You also get to show off a lot of timber, the assembly process is great, and as long as you keep the bushings straight, it's darn near fool-proof. I will say that on the center band side of the lower tube, instead of using the stock bushing, I use something a little bigger to turn it (the clip side bushing from the cap tube works good), and then something a little smaller than stock to give you clearance to round the edge over. The way the stock bushings are set up, you'll end up with a square, sharp edge on the tube that is supposed to meet up with the rounded bead of the lower tube fitting. By rounding over the lower tube at the top, you meet the round fitting with a rounded barrel, and it doesn't look out of place. This tip was posted by someone a while back, but I can't remember who, otherwise I'd give some kudos.
For pure customizability, the 7mm Euro is unmatched in my book. You have a lot of wood to play with, the little grub screws for making cumtom finial caps are easy to get, and the center band can be chucked altogether, allowing you infinite center joint diameter/embellishment options. The length of either the upper or lower tubes can really be tweaked a lot, but you need to keep in mind the length of the stock clip if you start shortening the upper tube. It's also a fairly safe alternative to the Slim, and will teach you about cutting tenons if you intend to build it stock.
Slimlines can also be altered with some creative tube shapes if you ditch the center band.
Those three are a good start.
