depends what the other person is using to be honest...

any combo can work with the right strategy

I can't say what the best army is, but i can say against what your group will do best

Recon will forbid any special force attack, and provide some nasty damages to any kind of infantry group, or groups without solid anti-inf damages. However, by putting Spec ops with Mechanised, you are doing the choic of having "one unit very good in one terrain" and the other "in another". It may give you flexibility, but it could give you contextual weakness too (In mountains, your spec ops won't cover with their bonus the malus of the recon/mech).

Your composition is very vulnerable to all kind of air attacks, by the way, and will be destroyed swiftly by successful gunship runs.

In other words, i think your composition is very good against a ground-based ennemy that has too many motorised infantries and heavy anti-armored units, with a geography composed of plains and forest.

1st, if you use it only for recon, you can use single CRV, if you want be possibl to fight insurgents or single enemy unit your stack is alredy ok, but you must have luck enemy cant react quickly and sending reinforcement, wich would kill your 3 battalione strong reconnaisance regiment. For better survability you need use unit of 5 regiments/battaliones.
However you must see that recon unit has high speed abilities, also dont mix it with low speed units. Speedupgraded tank would be nice, so you get a HP boost, anti infantry boost and none speed debuffs.

In new rounds CRV is really weak.
Also what exactly do you mean with your reconaissance? lightning areas for your arties?, lightning neighbour provinces in small areas? Really if you use it only for recon, you have behind every recon unit main battle group, which can go in battle within max. 2 hours? Then you dont need specifical combos. Your 3er stack is enough, even double CRV would be enough.

For deep raids you should have full strength of 10 Units. For flanking at least 5er.
And sure for whole war you need airforce.

My favorite Recon Unit composition is 1 UAV, cheap to make and I don't care if it dies, I will make a new one.

I was going to say something similar. uav can get you info quite quickly, but I usually will do a multi level recon of an enemy (provided of course i have time, and how far in the game it is). My base is specfor. I will send a stack of 3 in, they can helo in to get close, then drive where i dont' have coverage. I use three so that if i run into a stack of something that isn't just inf (3 SF's can take out a lot of infantry, esp if you use terrain right) and if i hit something armored or just big, i can have a unit rear guard, while i split the stack and get two units away, then send in the main force. They did their job, found me a big target.

If they have a lot of coastal cities i will use a sub to prowl their coast and spot targets. Subs are cheap and don't have much in prereq to build them

and for air, i have to say that although uav is cheap, it takes a lot in resources to get the prereq buildings, for a unit that can only find things, and then usually can't be reused as it is dead. So my go to on air surveillance is awacs for the long range, but close up detail i send in ASW helo. they have radar, can pickup infantry units which awacs doesn't, is fairly fast and cheap and can find subs, which uav can't. plus it has anti ship capabilities so if you spot an enemy trying to land forces on your doorstep, he can actually attack them.