I think the best HD rifle/carbine is what I use (duh!), but here's what I use and the rationale behind it. Just my opinion.
Saiga in 7.62x39mm.
Price: Can be found for $235. That makes it affordable, and not a terrible loss if stolen or confiscated by the police after a defensive use.
Speed:
a] Time to deployment is fast. There are no gadgets, no optics, no switches, no nonsense. Just a nice, long, easy to grab charging handle that has a long pull.
b] Semi-automatic. This is far above and beyond superior to any other action/mode with the exception of select-fire. Under stress, it is difficult to work a lever or pump a shotgun.
Reliability: 7.62x39mm cartridge is tapered making it an easy feeding round. It is also easy extracting. More importantly, the Saiga is a true AK-actioned carbine. This is the most reliable action available. The Saiga is a modern AK action, so the parts are well machined and clean and well finished. This is a fine carbine. Reliability is #1 above and beyond all other factors. Going BANG is king for HD.
Effectiveness: Semi-automatic fire coupled with the 7.62x39mm's power is superior to just about anything you can use for HD with the exception of a .308 or a 12ga, but you don't have the disadvantage those do in recoil. .30 carbine, .223, and most others do not match this power. Capacity is 10rds by default, but you can modify an AK mag to fit very easily with no detriment to reliability.
Penetration: I view penetration power as a benefit, not a liability. The very hard-jacketed Russian bullets, weighing 123gr and being FMJ is far better than the .223 at defeating barriers and THEN still delivering a lethal or incapacitating blow. Barriers such as interior walls, doors and furniture or cabinets. I can link you to two different defensive situations where the victim shot through a door to stop the attacker.
Ergonomics: Pretty decent. It has a traditional rifle stock, not a pistol grip - so it handles a lot more like a shotgun. It is a great point and shoot carbine for HD ranges. I can lift the rifle and do a quick snap shot and hit head shots at 25 yards consistently. That's much further than any HD range unless you have a large home. Rapid aimed fire is easily COM hits to 100 yards. It is a 16" carbine, so it won't be any longer than most other HD carbine choices. It's not as light as an M1 Carbine, but it's not heavy enough to be a concern. Weight in my opinion is a total non-issue when it comes to HD since engagement time will not be long. IE, you're not going to hump the rifle on a 10 mile march.
Overall, it is an affordable, powerful, fast, reliable, effective, good handling, easy to use HD choice. There's everything to like about it, nothing not to like. I overlooked it for years until I did a truly down to Earth serious appraisal of what MY HD *needs* are and what constitutes good defensive gear, rather what my *wants* are. Once I threw away all the stupid tactical-ninja ideas that filled my head from reading forums and instead used my own ideas formed from reading real defensive situations, that's the conclusion I've come to. I want more speed than levers or pumps but without sacrificing reliability. I also don't want to sacrifice much power, but do not want excessive recoil to slow me down. Most of the defensive situations I've read about were under 5 shots. That's not to say I don't need a high-capacity magazine, I'm a HUGE advocate of that, but it shouldn't be the single most important determining factor. If you consider a lever or a pump, they are already behind a Saiga in round count.
I believe the Saiga is about 10x more robust, tougher and higher quality than a Kel-Tec carbine. Handle both (I have) and use both and you'll agree. It has quality reliable magazines which is a benefit over the M1 Carbine which can be finicky with magazines, including USGI.
The only concern is to make sure you choose some quality 7.62x39mm. If you do not trust Russian ammo, domestic ammo is made and is high quality. However, I've never, ever had a single issue with Russian ammo of any kind - but I do prefer Barnaul over Wolf since it is mil-spec and a bit hotter. Just like a pistol, it is highly advised to fire at least 200 rounds through the defensive firearm to determine reliability of the firearm, the ammo and the magazine as a "package"...