For me, it's without a doubt the AR15 in either 5.56 or .300AAC. Both calibers have increased effectiveness over the .30 carbine, both can be had with ammo that doesn't over penetrate.
I don't know, nor have I even heard of any agency, military, etc. still using M1Cartbines. During the early 80s when stationed in South Korea, the police had them, but I think even they've moved on. Bottom line, it had it's day. IMHO, take the nostalgia out of it, and it's simply not the most effective tool for the job.
And honestly that's what really matters, the most efficient/effective tool for the job.
The AR15 is just too flexible a platform; adjustable LOP for family members of different stature, and ease of mounting an optic & white light, highly effective calibers with ammo easily available. Personally after taking multiple defensive shooting classes, a couple of which were low-light, I won't maintain a firearm for HD without a white light on it. Just not worth the risk of screwing up TGT ID and threat level.
From a training stand point, the majority of carbine classes are geared towards the AR15, that's the gun most of the instructors have experience with. From a practice standpoint, AR15s come in .22LR, and 9mm, both economical to practice with. .22LR conversion units are cheap and reliable. I can spend (and do) an afternoon running CQB drills and not break the bank, nor screw up my AR500 steel.
My then 11 year old with a S&W M&P15-22 set up similar to my HD carbine, same basic function as the real deal:
My Colt AR6951 in 9mm, also set up similar to my HD AR15 (I also use it for PCC matches), my wife easily can run a plate rack with it:
Colt LE6920 as it sits in V-Line vault: