Starting out with the Tour since this has been the centre of my cycling interest in the early years.. and still is. So here are some stages that I think would be worthy of consideration, not necessarily as the best mountain stages, but deserving of lower spots and someone that I personally will remember:
Luz Ardiden 2003 - after Ullrich attacked on Tourmalet and Armstrong was somewhat in trouble, LA showed who was boss. One of the most memorable stages from my childhood.
Alpe d Huez 2003 - Mayo incredible
Ax-3-Domaines 2005 - Vino kamikaze and strange T-mobile tactics
La Toussuire 2006 - Rasmussen epic, Landis crack
Morzine 2006 - Landis *** move, one of the best, definitely.
Aubisque 2007 was a proper queen stage, Tignes deserves a mention due to Rasmussen
Hautacam 2008 - good racing
Le Grand Bornand 2009 - great stage
Saint Jean de Maurienne 2010 - Contador and Schleck tandemn, epicness.
Galiber 2011- Masterfull victory by Schleck over Agnel (or Izoard) and win at Galibier
Alpe d Huez 2011 - Altho the following stage was better imo, this one was also rather crazy.
Annecy Semnoz 2013 - great duel, high tempo.
PSM 2015 - much anticipated showdown
Giro:
I haven't watched nearly as much Giro apart from the past 5-6 years or so, but here are the stages I enjoyed.
Sestriere 2005 - A stage which has been talked about on this forum, so had to watch it
Aprica 2006 - Robobasso, pure domination
Tre Cime 2007 - Saunier Duval domination, I miss the days with Piepoli, Simoni and Ricco
Briancon 2007 - something tells me that stage was good, can't really remember
Aprica 2010 - one of the best stages, clearly
Zoncolan 2010 - Robo
Gardeccia 2011 - one of the best, if not the best i have watched
Val Martello 2014 - this stage will always be remembered, but not because of Quintana's win
Aprica 2015 - again this stage delivered some amazing racing
Vuelta:
Same as the Giro, but I have probably watched even less Vuelta from 2005-2010.
Valgrande Pajeres 2005 - Heras and Liberty dominating the lone russian
Angliru 2008 - Contador dominating Purito and Bala
Fuente Dé 2012 - oldschool racing
Cuitu Negru 2012 - for some reasons, I really enjoyed this stage
Angliru 2013 - Honer full Froome ***
Ancares 2014 - the Tour-showdown which all had waited for
Cercedilla 2015 - really good stage
(Els Cortals) 2015 - one of the only good stages along with Cercedilla last year, but those 2 delivered, especially the above
Just fire away, I think it should be possible to create a poll after we have found the best candidates!