I like Trotsky, despite his aggressions and obsessions and general weird-ness. Considering that he was, after Lenin, the most influential figure both in carrying out the actual revolution and in "consolidating Bolshevik power" (I hate that little phrase-- I drastically overuse it in essays) up to the mid-1920s, I think in terms of achivements and pre-Stalin-manoeuvers position in the party, he was the most legitimate successor to Lenin.
However, once Stalin became more prominent in the lead-up to Lenin's death, the dramatic drop in Trotsky's power and popularity pretty much established him as a 'Nigel No-Friends' (I haven't used that expression in years) by 1929, so you could say that even if he was the most legitimate (which you could argue against anyway), he wasn't the most probable or was incapable of maintaining the position or something like that.
ie you could argue either way. Which is always the case in modern, which frustrates me incredibly when studying or learning, but is actually quite nice when faced with an essay in an exam that you really don't have much info to answer with.