Defenders

2 Gyrarado, 1 Dragonite, 2 Vaporeon, 1 Exeggcutor

Reason: highest CP my my lists.
It is really competitive to get a spot at the gym. You have to be min 2700+ and i am only at level 30

Dragonite
Snorlax
Gyrados
Vape
Exeggutor
Arcanine

Hmmm I should probably pull up another couple vapes

Gyarados, Rhydon, Exegguctor

• all have big weaknesses, but combined with their high cp this means I can take over a gym, and quickly prestige an extra couple of slots, hopefully quickly building a lv5 gym with me at the top...

Snorlax, Lapras, Chansey - probably the three biggest pains to attack. None are unbeatable but all take time and effort for a lot of lower level players.

Simple;

Snorlax
Snorlax
Snorlax
Snorlax
Snorlax

and Snorlax

Slowbro 2287- still a champ
Dragonite 3157- easy to prestige, ice is rare in area
Exeggutor 2666-easy to prestige, still hits hard
Machamp 2200ish- looks badass?
Multiple Snorlax 2700-3064-for far away gyms, it's cold

I have had these 6 mons firmly placed in different Level 10 gyms for the last 2 to 3 weeks:

• 2670 CP Vaporeon
• 2669 CP Snorlax
• 2483 CP Exeggutor
• 2326 CP Exeggutor
• 2147 CP Slowbro
• 2041 CP Poliwrath

My other defenders, currently not placed in any gyms, are:

• 2633 CP Vaporeon
• 2382 CP Lapras (I like to keep it available for dragons)
• 2368 CP Exeggutor
• 2101 CP Vaporeon

I am level 34:
Snorlax
Snorlax
Snorlax
Lapras
Vape (WP)
Vape (WP)

1. Snorlax (ZH/BS)
2. Snorlax (ZH/HB)
3. Lapras (IS/IB)
4. Dragonite (SW/DP)
5. Exeggutor (C/P)
6. Slowbro (C/P)

Because in my town you don't have to worry too much about CP of your Pokémon because there's probably around 10 Team Valor players so there will pretty much always be a spot open for you in a gym. That's why I avoid using Rhydon and Gyarados even though they're still higher CP than my Lapras, Exeggutor or Slowbro. I avoid using Vaporeon too because most other players use mostly Vaporeon.

I have two type of gym defenders: one for showing off, one for real defending.
The first category includes my top attackers - Dragonites, Alakazam, Vaporeon(HP), Arcanine, and Exeggutor. It also includes Gyarados and some others. They don't make really good defenders but you'll feel very proud to have them on gyms.
The second category includes my two Snorlax (Z+E and Z+BS), Lapras(I+B), Chansey(1200+cp) and Vaporeon(WP). I usually do a 5-gym run, so they'll just do (actually you just need 4, the last one doesn't matter). Normally I don't put Chansey, for making other players' life easier, unless I'm dealing with some personal issues.

TOP DEFENDERS:
Snorlax ZH/BS, 3027 CP powered;
Lapras IS/Blizz, 2617 CP powered;
Dragonite SW/DC, 3171 CP powered;
Dragonite SW/DP, 2867 CP not powered;
Dragonite SW/HB, 2826 CP not powered;
Vaporeon WG/WP, 2554 CP not powered.

TOP ATTACKERS:
Dragonite DB/DP, 3219 CP powered;
Dragonite DB/DC, 2183 CP powering;
Dragonite DB/DP, 2379 CP not powered;
Gyarados B/HP, 2910 CP powered;
Vaporeon WG/HP, 2825 CP powered;
Exeggutor ZH/SoB, 2646 CP powered;
Jolteon TS/TB, 2459 CP powered;
Gengar SC/SB, 2341 CP powered;
Alakazam PS/P, 2145 CP powering;
Cloyster FB/Blizz, 2126 CP powering.

By CP:

Dragonite, 3501 CP (14-13-15 // 93%) SW/DC
Dragonite, 3497 CP (13-14-15 // 93%) SW/HB
Dragonite, 3479 CP (15-14-13 // 93%) SW/DP
Dragonite, 3462 CP (14-15-12 // 91%) SW/DC
Dragonite 3441 CP, (14-15-15 // 98%) SW/HB
Dragonite 3430 CP (11-12-15 // 84%) SW/HB
Snorlax 3251 CP (15-14-15 // 98%) ZH/HB (bonus 7th)

Coolness factors:trophies 1700 Kabutops (coolness factor), 2004 Venasaur, 2180 Charizard (this is a game for kids and adults I like to put up those 3 to spotlight some awesome Pokémon)

Seriousness gym buildings: 2 Snorlax & 1 Dragonite

(Possibly 1 2700 Garados for gym placement)