The best tactics I've found...

Lafortezza

I was getting bored of my 4-4-2 formation, so I checked out the SIgames site and saw this tactic recommended. I downloaded it and tried it out.

When I first loaded it up and had a look I was like wtf? It looks weird and the player orders seem silly but the results are amazing.

Defence: 4 at the back but with no man marking, and back line set to push up.

DMC
2 MC
AMR/AML
FC

You should just me able to slot your players into the formation and play with immediate results, no need to go fiddling with player orders unless you need to.
With the player orders set the way they are I found I get loads more goals from the midfield, which is what I was lacking.
I'll try to put up some screenshots tomorrow or the next day.

Download tactic here

Btw I didn't make the tactic, some poster called jep on the Sigames forums did.

Boro

Sounds identical to the tactic i made up a good while back. I was actually attempting to replicate the man utd/chelsea tactics (in RL) and it worked very well. If you give one of the MC's a free role (if he is suited to it) and focus passing on the wings, set your striker as 'target man' and aim all crosses at him, it works really well. Very solid defense and surprisingly good attack.

Lafortezza

Only thing I found was that your players, especially the AMR and AML, get knackered very quickly. I have a huge squad with Juve so I'm able to send out a nearly full strength team every game. league+CL+cup = tired.

GadgetFiend

I've been using a slight variation of this with Norwich that got me promoted straight away..

GK
DR DL DC DC
DMC

Have a three man centre midfield with the two side midfielders having arrows to AMR and AML positions (good to have AMLC and AMRC players for this)

and 2 forwards, one a striker (SC) and one a forward (FC)

working a charm so far

but at MK Dons i was always a fan of the one target man up front

Dalli.S

Am playing with

3 cen def
2 cen mid (usually mc qand dmc but vary quite alot)
left and right mids( usually aml/r or fl/r) pushing up
1 support man behind the front 2 (free role)
and 2 up front

make sense??

works wonders do concede a few on the break but score alot more.

have the right players works a treat.

MrJoeSoap

I use a standard 4-4-2 with both centre mids dropped deep and both wingers getting forward plenty and it has worked a treat. It's all a matter of finding something that works for you and keeping it.

rondeco

the diamond formation is class.

4 defenders as standard
1 dcm
2 cm
1 am
2 strikers

you can really get a good return with a good attacking midfielder and if u can get a dcm with a great shot watch him bang em in all day!

Johnny_Trotter

I go for a 2-3-2-1-2 formation. needs a top quality DMC though, and the wingbacks help to score. central midfielders are given a lot of freedom then, again highlighting the need for a strong DMC

Pepe LeFrits

If you can get the right players who are flexible, my favourite formation now is 4-2-3-1 -> flat back four, 2 DMCs, AML, AMC and AMR, and SC. When the DMCs are of the creative playmaker variety (ie a Pirlo), it works superbly. The defence is kept absolutely watertight, the 3 AMs are very effective on the counter, and the formation can switch from defence to attack and attack to defence very quickly. If it isn't working, it can easily be switched to a wide or narrow midfield diamond or a standard 4-4-2.

Lafortezza

Actually one thing I've noticed about the formation I posted at the top is that your DMC is key. If you have two decent attacking MC/AMCs inthe middle of the field then your DMC needs to have high workrate/tackling/teamwork/stamina. You need him to be making lots of tackles and pressuring the opposition more than getting forward to attack.
I have Freddy Guarin and Lorik Cana as my DMCs, both have good stats for Long Shots, and also for Long Throws. Guarin has got about 8 assists per season solely from long throws to my DC's on the edge of the 6 yard box.

Zapp Brannigan

I play a 4-1-3-2 and I'll never go back.

My sole MC is the anchor (its usually Ballack, Viera or Alonso) and my AML AMR and AMC (C Ronaldo, Joaquin and Rosicky respectively) just tear up the opponents defense to let my 2 strikers do their work (Tevez and Fred)