I think "best" is relative of course. I think an argument could be made for the simplicity and relatively high HP of an old 448 for instance.
Buttttttttttt if we're looking at capability I think it's either 4223 or the 4120, I'll talk about each independently.
If a 4020 is a run of the mill Silverado then the 4223 PS/PTO is most certainly a Sierra Denali. The power steering with improved tighter turning radius, the foot control, under the seat PTO, more fuel efficient and more powerful engine... I really can't get over how good the foot control is on the 4200 series tractors. Anyone who hasn't experienced it should when they get the chance. And it's also nice when jumping back and forth between an Ingersoll loader and a GT. It's happened more than once that I've jumped on one of my hand control tractors after getting off a foot control and just thrown it in to full Forward by mistake hahaha. That being said, if you drop the belly pan on a foot control power steering tractor... wow. 10lbs of sh!t in a 5lb sack for sure. Much more crowded than the foot control and power steering combo on the loaders, especially with the normal GT frame instead of the wider loader frame. The Vanguard is a necessary evil. My Onans start much easier and run smoother, but I do like the much much better fuel economy of the Vanguard, as well as the slightly greater HP/TQ (Onan: 20hp/29.2lb-ft, Vanguard: 23hp/33.6lb-ft). If the 24hp Onan was ever offered in the tractors I would take that over the Briggs and deal with the worse fuel consumption for the tradeoff in ease of starting and smooth running. I have to say, it is nice being able to stop by a Lawn and Garden store and pick up engine parts / filters / etc. instead of having a usually longer lead time and shipping with Onan parts.
Using my previous metaphor as a starting point, the 4120 is a gasser 2500 Silverado LT. A little more creature comforts than your average tractor with the addition of power steering, but it suffers from a worse turning radius as a result. The hydraulic PTO standard means it always comes properly equipped for hydraulic implements. The engine doesn't have the power that the later Vanguards had, but it's a good motor and torquey. More importantly, the hydraulic system is substantially more capable than the normal tractors. The higher pressure relief and flow rate allows them to transmit ~18hp hydraulically, as opposed to the normal tractors' ~12hp, a 50% increase! And the tractor's cooling package is revised to handle the extra heat. It gets a dedicated large hydraulic cooler up front with a mechanical fan and ducting, as opposed to the "normal" 4 digit tractors which have a small cooler located in the tower which relies on the engine flywheel fan to pull air through. This relocation of the hydraulic cooler combined with the dual section pump being used for power steering (eliminating the need for the huge 1.5gpm diverter valve that most PS tractors have) means that the underside of the tractor and the inside of the tower are pretty uncomplicated and easy to service. One "downside" to the 4100 series tractors was their lack of a mechanical PTO clutch. This means all attachments had to be driven by the hydraulic PTO, increasing the cost of the attachments and reducing compatibility. It didn't help that the new higher flow rate pump meant some hydraulic attachments designed for the "normal" tractors couldn't be used with the 4100 series (I.E. wood chipper, 3pt mower deck, bagger, brush hog), for risk of overspeeding them and causing "rapid unplanned disassembly" as they say, which further reduced attachment compatibility. For a product line whose owners were used to keeping their attachments as investments and just upgrading the tractor, this wasn't a great look, and ultimately the 4100 series was only sold for two years. That doesn't change the fact that I still think they were one of the best! Ingersoll had thrown itself full into the capability of hydraulic power when choosing not to have a shaft PTO on the early tractors, the 4100 series is the pinnacle of hydraulic capability for Ingersoll. If anything I wish they'd doubled down, increased flow rate and horsepower, and expanded the types of hydraulic attachments they sold. The loaders should have definitely gone to the high flow hydraulic pump that the 4100 series had, especially with the introduction of the LQA down the line... My only complaint about my 4120 is the lack of foot control haha.