Tyrion: Game of Thrones' Greatest Lannister-Killer
The most straightforward interpretation of the prophecy is that Tyrion will be the Valonqar - which is also the most popular fan theory. Tyrion is her little brother, both in age and in physical size. He's also the (self-ascribed) "greatest Lannister killer of our time." Tyrion has been blamed by his father, Tywin, for killing his mother when she gave birth to him, thus giving him his first Lannister kill. Since then, he has also killed Tywin with a bolt through the heart, and he has abandoned his family to support their rival, Daenerys.
Furthermore, Cersei hates Tyrion, and he hates her. Cersei herself has become convinced that Tyrion is the Valonqar, and she put a bounty on his head. In terms of motivation, Tyrion certainly has enough reasons to want his sister dead, and he's shown that he's more than capable of killing - and of choking someone to death, which is how he killed his lover, Shae, when he found her in his father's bed.
This is a simple theory, but not necessarily the most likely one. Although Tyrion has plenty of motivation for killing Cersei, he would find it difficult to get close enough to her to do so, especially as she already suspects that he will try, and he would have to take her by surprise in order to physically overpower her. Tyrion may hate her, but he may not hate her enough to abandon his queen and position as Hand in order to head to King's Landing on a murderous vendetta.
Jaime Becomes Kinslayer And Kingslayer
The other obvious character who could be the Valonqar is Cersei's other brother, Jaime Lannister. Jaime may be her twin, but he was born moments after her, thus making him a "younger" brother in the most technical sense. Jaime is also becoming increasingly unhappy with Cersei, according to actor Nikolaj Coster-Waldau, and shocked by her behavior. He still loves her, but over the course of the Game of Thrones series, audiences have seen that love go from quite simple and devoted to something much more complicated. It's difficult to imagine the current Jaime pushing a boy off a tower for seeing them together.
Jaime is grieving the loss of his children, and may blame Cersei for Tommen's suicide - and he has some potentially complex feelings for Brienne, not to mention her straightforward honor and loyalty, which is in stark contrast to Cersei's scheming ways. Already a Kingslayer, Jaime may find himself forced to kill another ruler a second time. Like the Mad King, Cersei has started killing people en masse to get what she wants, using wildfire to do so, and if she continues to become more unhinged, Jaime may well feel that he has to kill her. However, there are some arguments against this theory, too. Appalled as he may be at Cersei's behavior, Jaime does love her, and she's currently carrying his child. It's difficult to imagine that their relationship has deteriorated so far that he would actually kill her with his bare hands, but it's possible nonetheless.
Arya The Assassin And Her List Of Names
Next up on the possibilities list is Arya Stark. Although she's not Cersei's younger brother, she may still fit the Valonqar definition. In High Valyrian, many words are gender-neutral (like the word for dragon), which could mean that the Valonqar could be any gender. It also isn't entirely necessary that the Valonqar is Cersei's younger sibling, but simply any younger sibling - which Arya is, with multiple older brothers and sisters.
If we accept that the Valonqar could be any younger sibling, then Arya makes a lot of sense. She has plenty of reasons for wanting Cersei dead; Cersei is the first name on Arya's kill list after all, earning the top spot for her part in the death of Ned Stark. Revenge isn't her only motivation, though. Cersei is the primary enemy of the Starks, and the Jon/Dany alliance. Taking Cersei out would be a service to Arya's family and the woman that they want to see on the throne.
In addition, Arya is physically extremely capable of killing the Queen. She has proven to be a ruthless killer time and time again, and her training with the Faceless Men gives her the ability to sneak into King's Landing and murder the Queen relatively easily. She also has no other demands on her time - she is not making a play for the throne or acting as Wardeness of the North, so there's nothing to stop her heading to the Capital to take out the Queen.
Other Possible Valonqars
There are a couple of other options, although these tend to be less popular as far as fan theories go. Sandor Clegane, the Hound, is another "younger brother" - to Gregor, the Mountain - that hates Cersei and would be more than capable of killing her. It's also likely that the Hound will head to King's Landing at some point in Game of Thrones season 8, specifically for the highly anticipated CleganeBowl showdown with the Mountain. Of course, if audiences accept that the Valonqar prophecy could refer to any younger sibling in Westeros, this also means that pretty much any character who hates Cersei and isn't a firstborn could fulfill it. Another Stark child might; Jon Snow might. This interpretation of Maggy's words leaves the playing field pretty wide open.