If the epic space opera experience is what you're after then definitely play all three. Seeing the whole thing out from start to finish, seeing how your different choices and relationships with different characters play out, is a big part of what made most people fans of the series. Start from the middle and you miss out on a lot of that stuff.
A lot of people say ME2 is their favourite, and that's fine, but in story terms it's probably the least 'epic' of the series - it's more like a whole series of smaller character pieces. Lots of fun, just not necessarily epic.