Depends on what kind of environment you are going to be fighting in. Plains? Woods? Dense forests? Desert?
Do you have field support? How long will you need to be expected to use it? Is the enemy likely going to wear body armour?
All these affect. There is no one gun for all situations.
Personally I'd go for an AK-47 derivative, thinking of the RK-95. Used by Finnish Defense Forces, it has the base AK-47 structure but not made of wood or plastics. Has a folding stock for urban combat, rails for NW-scope and can use rifle-grenades.
A bit on the heavier side in comparison to many smaller caliber assault rifles, but that is to be expected of a rifle based on the 7.62 round. Kickback is also a bit of a b***h, making the automatic mode of the weapon hit mostly air beyond point-blank range. While prone the accuracy is greatly improved, but not up to LMG/MG standards. Ammo does become an issue, as with heavier ammo than many other assault rifles, ammo conservation is somewhat of a problem in prolonged firefights. Or alternatively adds to the overall weigh. But RK-95 is meant primarily for single-fire mode, there is no getting around that.
It has the ruggedness and easy maintainability of the AK. The round is enough to penetrate most body armour and one shot does kill despite not having the speed of a 5.56mm or other smaller rounds. Effective killing range with iron sight is up to 400m.
The round also plainly ignores brick walls and small trees, keeping it's course fairly close to original trajectory after such an impact, quite unlike the 5.56mm.
A point in favour of most AK-derivatives is the easy with which it can be trained. You could teach all essentials, including basic maintenance and field-stripping, in four hours to your average 12 year old. Quite unlike many other rifles, including the M-16. I've done field-stripping on the RK-95 and then re-assembling it straight afterwards in under 30 seconds from redy to fire, to field-stripped to re-assembled to ready to fire.
30 seconds. It's just that simple. And that gives the reliability, the only moving parts are the gas-piston assembly and the trigger.
So, has punching power, highly reliable, accurate, easy to maintain and some modularity. If I was told I was going to a place I knew nothing about and told to prepare for combat, I'd take this.
The fact that I'm trained with it has nothing to do with me choosing it, nosirree
Normal infantry round penetration test for an identical rifle. Materials are: 20cm of wood (actual penetration capability while retaining lethal momentum is over 50cm), 13cm brick (standard house wall brick), 29cm concrete block (commonly used for building foundations), 8mm steel plate and lastly, the standard sandbag (23cm).
Remember, no AP-bullets, just plain standard infantry rounds.
EDIT: Here is a guy doing the field stripping and reassembling in 19 seconds flat. A bit messy with the parts, but done properly from start to finish. If you missed the stripping part, don't worry because it was
fast, he was done with it in less than 4 seconds.