I'm having the same problem as MXD - having a lot of trouble separating the 11-19 defencemen for this vote.
I've gone through the pros and cons of each candidate, so I might as well post them here.
King Clancy
Pros: Consistently finished high in Hart voting. Best defenceman in hockey for a few years before Eddie Shore emerged. Made a major impact on Toronto (positive) and Ottawa (negative) when he was traded. Very good scoring stats, and his defensive play was praised by contemporaries. Great team player and leader, with a single-minded focus on winning.
Cons: Rarely placed on all-time teams (possibly overshadowed by contemporary Shore.)
Sprague Cleghorn
Pros: Played for a very long time at a high level - over 20 years total, and over 15 years as an elite defenceman. Frequently mentioned as the best defenceman in hockey earlier in his career, and had strong finishes in Hart voting later in his career. Had a big positive impact on every team he played for. Offensive numbers and reputation were very good, and he was probably even better defensively. Years later, many people remembered him as the best or among the best defenceman from hockey's early years.
Cons: Competition level - hockey was still developing to some degree, and the best players were in 2-3 different leagues. His violent play could have a negative effect on his own team at times, and he may have had his own agenda in looking to inflict punishment on opponents.
Paul Coffey
Pros: Magnificent offensive player - 2nd best scoring defenceman ever. Major contributor to a dynasty. Major contributor to Gretzky and Lemieux's high-scoring seasons. Strong Norris trophy record. Played a major role in multiple Canada Cups.
Cons: Defensive play was questionable, although he probably improved on that in big games during his peak. Later in his career, he was just poor defensively - plus-minus was bad in regular season and playoffs. Traded away several times in the second half of his career, and his teams frequently continued to win or had even more success after he left. Needed to play in a system optimized to his strengths, and was probably in the absolute best situation for him in Edmonton.
Tim Horton
Pros: Traditionally rated very highly - THN had him 42nd on their Top 100 list in 1997. Arguably the biggest contributor to a dynasty and the best playoff performer. Excellent defensively, and good offensively in an era and on a team that did not encourage offensive play from the blueline. Terrific longevity - while expansion may have helped him play to 44, he was a legitimate #1/elite defenceman until the age of 40.
Cons: Norris/All-star voting totals aren't as impressive as they could be, especially in an era with few elite offensive defencemen.
Brian Leetch
Pros: Outstanding offensive player. Great Conn Smythe playoff performance in 1994. Won two Norris trophies against tough competition. Was his team's best player for most of his career, and played big minutes in all situations.
Cons: Average defensively, and weaker than all/almost all other defencemen up for voting. His teams missed the playoffs for seven consecutive years. His performance dropped off in the second half of his career, especially defensively.
Al MacInnis
Pros: Extremely dangerous in the offensive zone and on the power play, one of the most valuable players ever in that situation. Excellent longevity, with an all-time great season at age 39, and consistently very good to excellent performance over 15 years. Great Conn Smythe playoff performance in 1989.
Cons: Started off as a defensive liability. While he developed into a better defender over his career, he was never elite defensively. Often didn't draw the shutdown minutes or first-unit penalty killing duties. Had injury problems and early playoff exits in much of his prime in the 1990s.
Brad Park
Pros: Great all-around defenceman, with no weaknesses. Was hands-down the second best defenceman of his era, behind only Bobby Orr, and would have won several Norris trophies were it not for Orr. Strong playoff performances.
Cons: Other than Orr, his competition at defence was relatively weak. Knee injuries slowed him later in his career and limited his longevity. Never won a Stanley Cup.
Pierre Pilote
Pros: Great offensive defenceman, best of his era. Norris trophy record is strong. Good reputation for defence and toughness. Great playoff performance in 1961.
Cons: Late developer, which limited his longevity. Top-level competition for Norris trophies may have been a little weak. Black Hawks underachieved in the playoffs, raising questions about Pilote's playoff performance.
Chris Pronger
Pros: Great all-around defenceman, who could impact the game at both ends. Played huge minutes in his prime in all situations. Outstanding peak, winning Hart trophy in an era where defencemen were rarely considered. Excellent playoff performances later in his career, with several great playoff runs. Played for multiple teams, made all of them better. While he had injuries in regular season, rarely missed playoff games.
Cons: Injury issues - missed a lot of regular season games. Had discipline issues earlier in his career. Norris/all-star record is OK, but he had relatively weak competition, because the NHL stopped developing great defencemen after he entered the league. (OTOH, his Norris/AS record would be better if not for injuries/missed games, so don't double-count the negatives) Why did he play for several teams?
Earl Seibert
Pros: 10 straight postseason all-star selections. Great size and toughness, combined with discipline and mobility - like an early Larry Robinson. Strong offensively and defensively.
Cons: Was he ever the best defenceman in hockey? Peak may be lacking compared to some others this round, although I admit I'm having a hard time getting a handle on him. Some of his later AS selections came against weaker competition.
Scott Stevens
Pros: Most important player on a dynasty, and among the best playoff performers of his era. Completely changed his game to fit the needs of his team. Great defensive defenceman, maybe the best in the league for a few years. Outstanding longevity. Combined physical intimidation and great defensive play with discipline, at least later in his career.
Cons: His offensive peak and defensive peak were mostly separate - in his later years as a defensive star, he didn't have the high point totals he had earlier in his career. (1993-94 was where they mostly co-incided, and it was a great season, but he wasn't quite at his defensive peak yet.) Never won a Norris trophy.