Who is my next best defender?

I'm Level 39 and I think the most difficult pokemon to beat Blissey, Snorlax, & Vaporeon. The rest is debatable. Tyranitar, Dragonite, Rhydon, and Gyarados only gives your better placement.

Don't get rid of the Vape. Water Pulse is still good defensively. Placement-wise that Dragonite is your best bet, but all around good defender would be between that and that WG/AT Vaporeon. I'd go with D-nite because placement is really all that truly matters to most

the Exeggutor is definitely your next best defender from that list, but if he's super valuable to you as an attacker, i would keep him in that role. The Dragonite has a suboptimal moveset for attack, so I would just stick him in gyms -- he's expendable.

Wait lol what are the IVs don't transfer anything until you know we can't change movesets. And it doesn't matter bro you need to level up past level 30 to have consistent defenders in the gym ( for this current gym meta). But it should 100% be your Dragonite. Work on maxing that sucker out! It has a pretty high CP even at level 27! Should honestly be around 27-800??

I would say exeggcutor by a slight margin. Don't put him next to another exeggcutor if Dragonite gives you the same spot, though.
On a side note, I don't recommend transferring any above-2000 vaporeon, especially if it has decent IV. WP is not a terrible enough move to negate the fact that vaporeon is awesome.

Exeggutor. confusion is imo the best quick attack for defenders. Solar beam is (even if dodged and only taking 25% damage) really good, and with confusion it can gain a high EPS

Unless there is bug or fire thing going on in your area ofc.

Blissey...I can't believe there has been any other answer. Stack a few 2600+ CP Blisseys in a gym and it becomes more difficult to take down than anything before Gen 2. Period. ...It sucks.

One thing to also consider is if the Vap with WP has high IVs, move effectiveness can change and bad moves can be great later...Dazzling Gleam used to suck, now, it's a monster charge move dealing 100 damage for only 50 energy.

With possible major changes down the line, it's wise to keep all movesets IMO. I'm so glad I maxed my pokemon storage earlier during the sale...

I would go with the dragonite since it's the only one you can spare that has a CP high enough that it might hold a place in a gym in your area. Of course, if you have the spare candy and dust to power up the AT vaporeon, and it has good IVs, that might be a better choice.

I'd stick the Vaporeon in next, then the TreeHead, then the Dragonite, and not so much worry about attacking them as much as prestiging them and having a decent mon to stick in it.

Don't send the vap to the prof. Even with water pulse it is among top attackers based on damage before it faints. And for attack the IV is not that important.
Yes, it will time out vs ssme lvl blissey, unlike at or hp, but will beat any other mon and from your list appears that he should be part of every attack lineup of yours, at lesst for now.