You might need to narrow down what you are looking for to get a lot of good answers. Mods covers a lot of territory. Personally, I like box mods and am starting to lean more towards the variable voltage mods like Little Sister and Big Brother. I've made myself a VV Puck which I really like with the new CE2 cartos. If you're interested in making something yourself, check out the Battery section in the Modders Forum, too many ideas floating around in there!
I don't have any tubular mods so my opinion on them isn't worth much, but I'm sure someone will pitch in here shortly.
My opinion, when looking at mods you need to start with battery life. If you don't mind carrying a couple of small
batteries with you, a small mod that uses 10440 batteries may be perfect as they are usually tiny. I have a mod made out of a lighter, very small. If you want something that lasts all day you might want to look at either multiple battery mods, or the big battery mods that use the 18650 batteries, some of those can last for days but they are usually huge.