What starter Air rifle generally best to start with?

I’d say it depends a bit.
When I (re-) started airgunning, I wanted to hit targets, spinners, cards and so on, mostly benchrest or from a bipod. CO2 was a good choice to start with, drop-in and shoot, no hassles, no fiddling, just enjoy. The gun that got me hooked back to this hobby was a Haemmerli Airmagnum 850 in .177.

On the PCP side I’m more than happy with my Weihrauch HW100T, absolutely accurate for bench rest or prone with a bipod. Took a little time though to find the right height scope mounts to have a repeatable and well established cheek weld.

If hunting, I’d rather choose a carbine to start with, good value for money over here (other side of the pond) would be the HW100K (attention, the K is important for “kurz” – short or “Karabiner” – carbine) PCP + a Hill pump to start with. Alternatively well pleased with the .22 Brocock Compatto. Carbines are in my eyes very versatile for all positions and less front-heavy than a full-length rifle.
Also like my HW97KT (carbine, thumbhole), but springers need more practice in the beginning to hit things which can make a beginner turn away from airguns.

A bullpup, though fine guns and the least front-heavy can be tricky for a beginner for hold, head placement, overall control. All depends on the person, the guns mentioned in Zonk’s post are for sure fine guns, I can confirm this for the Cricket of which I own one myself.

Important in my eyes is to try stocks and hold maybe even fire some at a good dealer, not every gun fits every frame and in the beginning, you miss experience to be aware of that.

After having several lower end air rifles I started off with a Benjamin Marauder PCP to see if I wanted to got that route. Then I was hooked into PCP airguns. After several later I settled down with my two favorite PCPs. .22 Cricket and .22 Vulcan. Both VERY accurate.