chisel tips are the bread and butter of soldering. soldering is about heat transfer. heat transfer is about surface area. so in general you want a tip that gives you as much surface area as possible to put in contact with your joint without heating things up that you don't want to heat up, ie, that aren't your joint. the diagram above nails it. you want your tip to make contact with BOTH objects involved in the joint, heat them up above 190C (63/37's melt point) and then feed solder to both surfaces. the flux in the solder will lead the way for the solder to lightly coat the surface of the joint and then isometrically cool into a shiny little bead or coating.
there are two more tips i keep around for special usage: first, the wedge tip that is flat on one side and rounded on the other (basically like a half chisel). these are slightly better for smd wipe soldering than chisels. second, i keep a long pointy tip around when literally nothing else will fit into the space around the joint. but 99% of the tip i have either an 8mm, 16mm, 24mm, or 32mm hakko chisel on (the t18-dxx where xx is the width of the chisel surface).
i don't bother with a hot knife tip because when i need a hot knife i blast an x-acto knife with a butane torch and literally cut things with the hot knife that results.
