this kind has been the only sort we have used:
http://www.tommeetippee.co.uk/product/first_cup_4_months___/
they are free flow (but the water doesn't pour out if it's tipped - unless of course elijah shakes and shakes it haha) so they need to suck a little bit but not as much as with non-spill cups.
eli started using it at about 5 months and has never had any trouble, you can fold the spout down too so they don't spill in your bag. i really like them and they fit in most sterilisers for little ones.
it saved us getting elijah back into using a bottle (used one for 3 months combined feeding) so he has water/juice/milk from that, and bf too, hopefully we are totally bypassing bottles
i would really highly recommend those cups, they are pretty cheap from tesco