First and foremost, the best position to start is the position that you’re most comfortable in. Regardless of anything else, if you’re not comfortable then you won’t be in a position to have your best race.
After this, it depends on what you’re trying to achieve, but my suggestion is always to start on the outside of the group. For instance, if the first turn is round to your left, start on the right hand side of the bunch. This is because everyone will gravitate towards the racing line – the shortest route around the course. If this means you have to swim an extra couple of metres but you don’t get caught in the melée, then that’s a fair trade-off.
Being at the side allows you to move forwards relatively simply. If you wait at the back but are then swimming faster than those in front, you still have to swim past kicking legs.
Finally, if you’re doing a deep-water start (as opposed to running start), kick your legs up behind you so you’re horizontal to start with. It will create your own pocket of space that no one will come in to, and means you can also get away quickly.