I'd rank him somewhere in the top four along with Apps, Conacher and Kennedy. It's hard to compare Kennedy with Conacher and especially Horton because their styles are quite different. I'd probably have Horton 4th and I'm undecided for the top three.
Horton earned a spot on six all-star teams and was a 6-time Norris candidate (all with Toronto). He's arguably the greatest defensive defenseman ever and, incredibly, was 2nd in playoff scoring in 1962 when the Leafs won the Cup. He was a member of 4 Stanley Cup-winning teams and was probably the #1 defenseman for all of them.
Conacher led the league in goals five times, and won two Art Rosses. He led the playoffs in goals one year, and points another time. Conacher has a retrospective Conn Smythe, was a five-time all-star and won one Stanley Cup. He was a 2-time Hart finalist. All of these accomplishments came with Toronto. He used his considerable strength to overpower opponents, though he didn't fight much.
It's hard to compare a high-scoring power forward and a shutdown defensemen but Apps and Kennedy are very similar in style and accomplishments.
Apps and Kennedy were good playmakers but Apps was the better scorer overall. He was runner-up for the Art Ross three times and finished in the top ten 6 times. Kennedy peaked at 4th and was in the top ten 4 times. Apps led the league in assists twice, Kennedy did it once.
Both recieved a fairly impressive number of awards. Apps was a five-time finalist for the Hart trophy (finishing in 2nd place three times and in 3rd place twice). Kennedy won one Hart trophy (though it was almost certainly a political selection) and was a finalist in three other years. Apps was a five-time all-star (two first-team selections); Kennedy was a three-time all-star (all on the second-team). Apps also has a Calder and a Byng, but I don't think those are very relevant.
Both were known as good defensive players and leaders. Kennedy was rougher and more physical, but Apps was known as a very strong player who wouldn't back down to anyone. Kennedy was a much better checker. Both led their teams to considerable playoff success. The Leafs won more Stanley Cups with Apps as a captain (3-2) but more Cups with Kennedy overall (5-3).
Both were top playoff performers. Apps has one retrospective Conn Smythe trophy, and Kennedy has three (tying Roy for the most all-time). Apps led the playoffs in goals, assists and points once each (all in different years). He finished in the top five in goals 5 times, assists 3 times and points 4 times. Kennedy led the playoffs in goals twice, assists once and points once (in three different years). He finished in the top five in goals 4 times, assists 4 times and points 5 times.
Overall, Kennedy and Apps are roughly equal to each other. Apps has slightly more regular season hardware and was a better scorer, but Kennedy's toughness, 2 extra Conn Smythes and better defense nearly offset that. Both rank extremely high in leadership and playoff scoring. Apps and Kennedy are nearly equals to each other, and to Horton and Conacher. All deserve serious consideration as the greatest Leaf of all-time.
It would be interesting to hear Leaf Lander's perspective on this.