iPads

Best is subjective. Only you can deteremine which model and specification will meet your needs. You can directly compare major features of the various models of iPad here:

iPad - Compare Models - Apple

In summary, the iPad9 is the entry-level iPad - principally aimed at the education market. At the other end of the scale, the iPad Pro models represent the pinnacle of performance and features - with a price to match. The iPad Air is a high performance mid-point, offering much of the “performance” of the iPad Pro - but lacks the advanced hardware features of the Pro models (such as orientation adaptive multi-speaker audio, advanced camera array, LiDAR sensor etc.). The iPad mini6 is the smallest iPad.

Whatever model of iPad you choose, you would be well advised to purchase as much internal storage as you can afford - as internal storage cannot be changed, upgraded or extended. The available internal storage is fixed for the entire life of the device. If you run out of internal storage for your documents, photos and other data, you will forever struggle and become a source of frustration. It is far better to have more storage than you need, than to need more storage than you have.

A good rule of thumb is to quantify how much data storage you will need - add a healthy margin - then double it. If you are unable to quantify your data storage needs, it becomes more difficult; whilst I cannot make definitive recommendations, personally I would council against purchase of any iPad with any less than 256GB storage - 512GB or greater offering a greater level of safety.  

Within the Apple Support Communities we see regular tales of woe and disappointment when storage requirements are exceeded, necessitating replacement of the device; by contrast, we never see complaint of having too much available storage. It is relatively easy to fill limited internal storage space; perhaps consider higher tiers of storage as providing room for growth as your needs develop or mature - potentially extending the useful usable life of the device. 

Consider also the functional differences between WiFi Only and WiFi+Cellular models of iPad. Only WiFi+Cellular models include GPS capabilities - which may impact any Apps or features that require accurate and consistent location data, such as Mapping. Purchase of a WiFi+Cellular model does not commit you to using Cellular connections, but provides this flexibility should it be needed.  WiFi Only models of iPad have no Cellular or GPS functions at all.

I hope this information and insight proves to be helpful.

All current iPad models all run the same full iPadOS and all the same apps from the Apple iOS App Store.

All current iPad models all either use the original, 1st or newer 2nd gen Apple Pencil.

( iPad 7th, 8th and 9th gen and iPad Mini 5 all use the original, 1st gen Apple Pencil )

( 2018, 2020 and the newest 2021 iPad Pro, iPad Mini 6 and 2020 iPad Air 4 models use the 2nd gen Apple Pencil )

So, iPads are more like cars, now.

You are paying for iPad optional features, now.

Do you want/need larger internal data storage space for data, like music. movies and photos/images and room for other types of data?

Do you need a larger or smaller iPad screen?

Do you/want/need higher color gamut display with Tru-tone room brightness calibration?

Do you need a larger screen with much more pixels that is much brighter, with a high resolution dymanimc ( XDR ) color gamut range?

Do you need/want an iPad model in a variety of colors?

Do you want/need a bonded, laminated display?

Do you want/need faster, more powerful internal CPU?

Do you want/need more internal processing RAM?

Do you want/need better iPad cameras?

Do you want/need better/more iPad speakers?

Do you want/need the improved, 2nd gen Apple Pencil?

Do you want/need to have the new Magic Keyboard portfolio for the 2018/2020 iPad Pro/iPad Air 4 models?

All of these options are what you need to decide if you really need, or not, and to decide to pay for accordingly.

The more iPad features you decide you need, the more you pay.

It all comes down to how much money you are willing to spend on iPads with additional, premium features.

You can now pretty much do everything on any current iPad model, now.

All current iPad models are sufficiently powerful enough, today, to do pretty much any task put to them now.

https://www.apple.com/ipad/compare/

Purchase the largest internal data storage capacity iPad model you can afford as no Apple mobile iOS/iPadOS devices have ANY internal expandable storage.

Good Luck to You!