The Legion of Super-Heroes has a history of rotating the leadership amongst its expansive membership. For a long time, the Legion leader was elected by the readership. Sometimes this led to rather surprising results, but always, the writers, usually Paul Levitz, have managed to make it work.
Sometimes the leader has only lasted for a few issues, once only for one issue. Occasionally, the leader will stick around for some time. It's these cases that make for characters that fans consider the prime candidate for Legion leader.
10 Dream Girl
In her time as leader, Dream Girl led the team against one of their greatest enemies, Darkseid. It earned her respect and defined her past being a superficial platinum blonde. Dream Girl is one of the smartest Legionnaires and different versions of her have used her precognitive abilities in more creative ways.
After her term was up, she served as Deputy Leader. She took over when Element Lad went missing. In this role, her priority was finding the missing Legionnaires, who returned on their own. Nevertheless, Dream Girl showed an administrative ability that added to her already admirable assets.
9 Wildfire
Wildfire's tenure as leader was a very tumultuous one. He was elected leader despite not winning the most votes because the winner, Superboy, was not a full-time Legionnaire. His brash personality also meant that the Legionnaire was prone to letting his emotions affect his decisions.
In his time as leader, two founders married and left, Chemical King died, Dawnstar joined, and Brainiac 5 went insane and unleashed a near-unbeatable monster on the Universe. In an attempt to defeat that monster, Wildfire destroyed Legion headquarters. Near the end of his time as a leader, this was almost poetic.
8 Polar Boy
Polar Boy's time as leader saw the team nearly fall apart. He lost several Legionnaires and saw a conspiracy of four members to act on their own to kill the Time Trapper in revenge. Nevertheless, his was a story of the young boy that succeeded in his goal of eventually joining, then leading the Legion.
It took many years and a sidetrack through the Legion of Substitute Heroes. Polar Boy tended to defer to older Legionnaires, and he failed to address the departures with recruiting new Legionnaires. He campaigned so heavily at first that when it came time for re-election, the Legionnaires voted to prohibit it. His legacy is one of the worst leaders the Legion had in its history.
7 Kid Quantum
Kid Quantum is the only person on this list that comes only from a rebooted Legion. Elected after the Legion reformed following the Blight invasion, she led the team against Darkseid and Universo.
Sadly, her tenure didn't end with an election, but with a catastrophe. In a crossover with the Teen Titans, the Legion was rebooted for a second time. Her team was lost in limbo until they returned in Legion of Three Worlds.
6 Element Lad
Element Lad's entire life is the Legion. It's been his only home since his entire planet was wiped out by pirates. After serving multiple terms as Deputy Leader, he finally got two terms of his own.
What puts him over other leaders in the 70s and 80s is the brief time that he took over after Lightning Lad resigned. That time gave an impression of how he would lead the Legion against some very dire threats. When he discovered that Shrinking Violet was replaced by an imposter, this impressed enough of his teammates that he was elected despite not even running.
5 Saturn Girl
Saturn Girl has a good reputation as a leader, despite first being elected due to less than honest methods. All of it was to prevent the death of any other Legionnaires. Unfortunately, Mon-El warned Lightning Lad who gave his life to save hers. She went on to serve two terms as Legion leader and did admirably.
She ran for Legion leader a couple more times and remained one of the stronger voices in the Legion. In other versions of the Legion, she's demonstrated those same capabilities, although her headstrong ways have sometimes proved incompatible with the responsibilities of the position.
4 Mon-El
One of the most powerful Legionnaires, Mon-El has also proved to be a capable leader. He has a serious demeanor and a sense of responsibility. On the cusp of the New 52, he was elected Legion leader despite having resigned to be the new Green Lantern. He managed to serve both roles, primarily to prevent Brainiac 5 from taking the office.
He served through most of the Legion's run into the New 52, but his term ended with not only the election of a new leader but the fall of the Legion against the Fatal Five.
3 Lighting Lad
Lightning Lad's first term as the leader was abbreviated by his resignation. In terms of issues, it was nearly four years long, one of the longest "years" in the original Legion run. He suffered a mental breakdown after his wife and four other Legionnaires went missing.
In the threeboot version, Lightning Lad got another chance and did all right. It was a tumultuous time for the Legion, but he did expand the membership in the middle of a crisis. As the series ended, he remained the leader, serving long enough to make him one of the longest-serving Legion leaders.
2 Invisible Kid (Lyle Norg)
Lyle Norg is probably the second smartest Legionnaire, behind only Brainiac 5. In the canon of the comics, his term as Legion leader was so tumultuous that he made terms only a half year afterward. His time as Legion leader lasted so long that when fans think of the Legion during the Silver Age, the leader that they think of is usually either Lyle or Saturn Girl.
Lyle also has the advantage of serving as the leader of a rebooted Legion at a time when it spanned two bi-weekly titles. This makes for him to have been the leader for a lot of issues of Legion series.
1 Cosmic Boy
No matter which version of the Legion it is, Cosmic Boy is the default leader. One of his defining characteristics is his ability to lead others. Every time the Legion is rebooted or relaunched, it seems that Cosmic Boy is the initial leader that the team starts with. This makes for a lot of issues of Cosmic Boy as the leader of the Legion of Super-Heroes.
He's such a powerful leader that his advice is often sought by the team's leader. During the "Five Years Later" Legion, Chameleon Boy even sought out Cosmic Boy to lead the team, even though he had lost his super-powers. This tenure lasted more than fifty issues, more than enough to put him at the top of this list.