What do you think the most effective secondary weapon is and why?

I personally don't think there is one truly dominant secondary for all situations. There is obviously a 'fitting' secondary for different game settings which may depend on the map layout, game mode, and what type of class you are going for.

For my general all out killing classes I tend to go with either the M93 Raffical w/ silencer, AA-12 w/ grip or PP2200 w/ silencer. I find that pulling out the silenced M93 while being ninja is really effective at taking down enemies quickly and still remaining unseen. If I'm on a more closer quarters map such as Favela, Karachi, Sub Base, etc...I find equipping a more close range weapon such as a shotgun works well due to the many corners, hallways and rooms. Of course some parts of these maps are more open, which is where you'll want to use a more effective weapon such as an AR/LMG. When entering a narrow hallway with limited sight or a room, switching to your shotgun is typically more effective as it doesn't require much aiming and only 1 shot.

In maps with more open settings, I find that secondaries are not as useful in as many situations but can still serve a purpose. I generally always have scavenger pro set-up so I'll stick to my primary but in case you want to go with another Perk such as Marathon to get around large maps such as Wasteland quicker, a secondary with a bit more range than a shotgun will usually be more effective. If you happen to run out of ammo or are using a non-silenced primary, switching to a silenced machine pistol when coming up on a group of enemies usually works out best. With this you won't alert the teammates of the person you kill as long as they are not in close by. But also remember that even if you have a silencer and take someone down, they will more than likely notice a skull nearby and be on the look out. Here is where Ninja Pro comes in handy so that you can't be heard or seen on HBS.

Then obviously for those classes dedicated to taking down harriers, chopper gunners, ac-130s...It's best to go with a rocket launcher with cold blooded and even ninja pro if you want to go all out stealthy so you don't get shot by someone on the ground while trying to take an aircraft down.

Wow, I didn't realize I was typing that much. I think I just discovered that coffee not only makes you more talkative in person but also with a keyboard

Beretta M92F, hands down.
On HC, I don't even need a primary, (but a UMP-SD is taken nonetheless) just a M92F with a Tactical Knife, Scavenger Pro, Cold-Blooded Pro, Ninja Pro.

Another class of mine I like with the M92F (UMP-SD Primary) is that with a Tact. Knife, Marathon, Lightweight, and Commando Pro.

the striker is the best. One shot kill good range and high mag. It could be a primary. I went 30-5 with it today.

I would say the model 1887 ... because they have good range for a shotgun,good damage, and are very good on akimbo (twice the power!)

I play a lot of HC HQ so i tend to use the thunper ot clear out the HQ before entry of right after an enemy capture