I prefer 15+10. It helps players (like me) who tend to get into time trouble. You can get a winning position right at the end and without increment you would just lose on time, that sucks. I don´t see a reason why it should improve the quality of the game like ChrisWainscott says though.
Definitely g30. In 15+10 games, all it does is allow you to play much longer in time pressure. In g30s, you've got a decent amount of time and usually shouldn't wind up in time pressure at all.
Out of this two only, which do you prefer and why?
Which do you think is better and why?