Which mon can Surprised gym attacker ?

If you don't care about CP, Charizard is probably gonna be the best pick of the bunch - Tentacruel, Omastar and Muk are better defenders but your movesets are pretty meh. Arcanine with WC could also be a fun surprise for Vapes.

Slowbro: Confusion + Ice Beam

Confusion hits hard and ice beam will surprise grass types + dragonites.

I prefer +15 attack and +15 stamina for my slowbros, if possible.

(If you can get slowbros lol)

These are not on your list but they are pretty good non-metagame defenders:

1. Slowbro: Confusion/Ice Beam
2. Lapras: Water Gun/Hydro
3. Polliwrath: Bubble/Ice Punch
4. Chansey: anything

Also agree with the post above that out of your list, Charizard looks the best candidate. But why would anyone want to power up an Air Slash/DC Charizard... it's crazy.

The problem with all these surprises:
You will take the bottom place in gym. So it should only be used in gyms with high turnover.

I add steelix, who is one of the toughest mons in game besides blissey - almost as tough as a snorlax. It has steel typing, and is week to fire, water, fight and ground. - But it can take surprisingly long to kill with a dragonite.

Put Scizor between vapes

Unless the attack is 1.56x (e.g., ice against Dragonite), there isn't much surprise because the lack of stab offsets the type advantage (such as Arcanine with WC).

lazymonkey, where are you located? Somewhere in US or Europe? It's so nice that many of you have gyms that can stand for days, weeks and months. My local scene only allows gyms to stay up at most a couple hours before they get taken down.

Of your list I would go Charizard personally.

This is one I like to use that carries a surprise...
https://pokemongo.gamepress.gg/45171/grass-fire

Starmie with HiddenPower Fire
Ninetails with Solarbeam

I would go with charizard and arcanine.

Piloswine with IC/SE, Victreebell with RL/LB and Lanturn CB/HP are also fun to place in the gym.
Biggest problem with those mons is that it makes prestiging harder.