movietickets.com
all of you should only use it. and if you go to a National Amusements theater (as opposed to some smaller local place, or AMC), you should actually go to nationalamusements.com
why? National Amusements, at the Cinema de lux chains (like the one I work at), have something called Director's Hall, which is essentially a rip-off (about $3.00 more than a regular ticket, all for only leather seats and you get to pick your seat at the box office). The problem with most places, is that they do NOT differentiate between the times for director's hall and regular showings. This has caused many a costumer to end up purchasing more expensive tickets because the show time they want to see ends up being a showing in the director's hall.
i don't think any other theater chain does this, so its likely any other site is just fine.
even then, I stick to movietickets.com, which is partly owned by National Amusements.