What is the best position during sex?
It's the position that you and your partner find immensely pleasurable at that moment in place and time.
The answer to your question is going to change based on where you are, how you're feeling, and who you're with. So you and your partner get to figure out what's working and what's not. If something feels awesome, then feel free to scream out, "YES! Do more of that!" And if whatever happening isn't happening, you can say, "Whoa, let's change it to something else."
For tips on how to communicate your desires and needs to your partner, check out: Be a Blabbermouth! The Whats, Whys and Hows of Talking About Sex With a Partner
But whatever position you pick, make sure you're playing it safely. More info on that: Safe, Sound & Sexy: A Safer Sex How-To