Which one should be my buddy now ?

It would depend on what you already have.

If you don't have a strong Blissey, then I think that would top the list for me. She can be a good defender and a good attacker.
If that is your only Tyranitar then that would be next on the list.

...and can I assume the other is a Lapras?
I am sure you already know it is capable of destrying a couple of DNites as it is, so this would be less of a priority.

So in this order...
Blissey
T-Rex
Lapras

...unless you already have a stable full of any one of them.

EDIT I did not see the pic before I posted...as you have a good Blissey, I would be tempted to work the T-Rex first.

I'd probably walk the Blissey. P/HB is very good if you don't need to take down every thing super quickly, should destroy 3-4 enemy pokemon. You seem to have enough defenders already and your best dragonite is in the gym.

Btw, your pokemon have really funny names, love the Snorlax name lol

I would say lapras... I dont know why just lapras is cool

Wait... you can have more than 10 mon's in gyms?

I'd say Lapras, simply as it's my favourite.

You have enough Pokemon in gyms so you don't need any more defenders which leaves attackers, so I'd power up Lapras if I lack a strong Dragonite counter otherwise Blissey.

IT Tyranitar isn't an amazing attacker into anything and it's advantage which is it's high max CP for gym placement is irrelevant in your case since you have over 10 Pokemon in gyms anyway.

If you don't need Lapras or Blissey, maybe there's a Pokemon you need for the Pokedex?

The answer to the question of who to buddy up with is always Blissey/Chansey.

I'd go with Tyranitar first.
The Lapras is already a good Dragonite killer at this level and the Blissey has (imo) the worst moveset for defense (I know psychic has worst theoretical dps, but it's hard to dodge especially when player is attacking straight through her pounds while HB is incredibly easy to dogde, even in this case), so I'd keep it as an attacker. The Tyranitar is more balanced, good generalist attacker, good defender and can reach an very high CP, so I'd go for it.