In your situation, I 'd choose Snorlax as it would add best value to the gym. When your opponents want to destroy your gym, no matter where you stand, you will be taken down. Recently I have trained against Blissies around 3050CP and I success most of the times. So I think you should power up your Blissey over 3100CP then she will be a real challenge for a lone trainer.
Tyranitar is even worse than Dragonite. After training with some Tyranitars, I won't invest my dust and effort for Tyranitars.
GamePress
Gym Placement Should Be By Product of Stats
With Blissey's addition to the roster this debate becomes even more relevant. Blissey is the best defender in the game right now because the most important thing to do to deter your opponents from taking it down is make it time consuming to do so. Therefore, for the interest of your group she is the best option to use in a gym if she's available. However, for my own sake if I level an allied gym to level 10 there are 3 pkmn I might put in it.
CP 2796 Blissey CP 2938 Snorlax CP 3190 DragoniteBlissey would likely be in a mid-range position (5th), Snorlax in high-range (3rd) and Dragonite would probably be the gym leader (1st). So for my own interests Dragonite would be the best choice, followed by Snorlax, followed by Blissey. For the groups interests it would be the other way around. If rankings were by product of stats as opposed to CP then the individuals best choice would line up more with the groups which I think would improve the gym system.
(Tyranitar would be even more of a "self-interest" choice than Dragonite.)
Report
Answers
Alongside your ideea I would make another change to make good defenders more relevant, I would revert the fight order:
Fight from top, drop from the bottom, so the mist fights will be against best defender also when you take the gym step by step (fight till blissey, repeat, reapet, blissey once, blissey gone, restart the sprint).
