It just depends on what you like and on the cigar.
A v-cut or a punch are both good because they leave more of the head intact, which makes the wrapper less likely to come off and leaves that nice, rounded cap, which I think feels good in your mouth (oh God, I know Godfather's gonna post below this now with some perverted comment because I just said that).
The straight cut, like a guillotine, double blade, or scissor provides, is better from a tighter cigar, giving you more space to draw air through, but can also make the wrapper unravel if you cut too far or the cap peels off.
So for me...I usually go for the straight cut, but if I get a cigar that I know has a very easy draw (like an Indian Tabac Limited Reserve, for example), I like a v-cut or a punch.
It all depends on the stogie.