Too early for such a poll, as usual, because the last race always gets overrepresented. But anyway. To me the tour wins because it had a good balance with some more memorable stages (The sheffield stage won by Nibali with some crazy public, the cobbles stage of course), MTFs but also spectacular downhill finishes (Oyonnax, Bagnères de Luchon), the best sprinters on show. Of course Contador and Froome's exit are a shame, but still a very good vintage.
Giro comes second for 2 reasons : 1/ is the Val Martello stage. A real game changer. GTs should have big game changers like this, a stage that breaks the dynamic, changes everything. 2/ Rolland/Hesjedal/Aru/Pozzo making it a race for that 3rd place finish, with attacks, movement from afar (mainly from Rolland and Hesjedal), etc. if even if we had lots of MTFs, they had real variety in how they were ridden.
Vuelta comes last because we always had sort of the same finish, 2012 style with the same 4 guys and slightly modified roles. Sure Aru won 2 stages, but it came from the 4 guys just watching each other. Contador was the strongest, well done, but it was a GT without big dramatic showdown, without that epic moment when you can see the riders's face crushed with fatigue, when survival was the order of the day. The tour had that with the Cobbles stage (just look at Nibali's and Contador's face !) and the Giro too with Val Martello. The Vuelta didn't. It was entertaining in its way, but this does not a great GT make.
Give it 2 years. Who will remember a single stage from this Vuelta ? While Val Martello and Arenberg will be remembered for a good while AND the end winners of the GT were the ones who toppled the table and won big on that day. On Arenberg Nibali put Contador 2mins30 behind him and effectively won the tour (I am one of those who believe Contador was never going to be able to take all this time back). At Val Martello Quintana broke Uran and won his Giro.
These are stuff of lore, stages that stay in the memories, with their part of debate/passion/trickery. Nothing of the sort on this Vuelta.