Nami has good poke and while she doesn't have a melee all in like Leo but between her Ult, Bubble, and AA enhancing slow she can lock down targets really well.
Morgana might be considered 'all in' but at the same time you don't have to fully commit every time you land a snare. A snare + pool can be a good bit of poke to push the enemy out of lane at the same time a snare is a good all in especially with her ult.
Sona has good poke and her E (speed boost) along with her E power chord (enemy slow) are good engage pre-6, post-6 she has a large AoE stun.
Lulu has good poke with her Glitterlance (her Q aka Slow) but her Polymorph and her ult can help her all in/engage when necessary.
Janna has great poke without retaliation as well. You E (shield) yourself which gives you bonus AD and AA the enemy. It will proc your spellthief damage and your extra AD will make the poke more potent on top of having bonus MS from your W so that you can get in and out quickly and you're shield will block the retaliation.
Personally I feel like Nami is the best option, but pretty much any 'mage support' with CC will be able to do both. Janna is the second best imo. She doesn't do quite as much damage but she has good lock down and has a really nice AA buff that will help her win trades in an all in.