All models of iPad have very similar specifications for battery autonomy - these being “up to” ten hours. You should be mindful of this wording - as “up to” ten hours is only applicable in very specific circumstances; high CPU load and/or high screen brightness may significantly increase battery utilisation - requiring recharge much earlier.
As for which iPad is “best” is subjective - and will be different between users. iPad models have different physical dimensions and weights - and differing performance. You can directly compare iPad models here:
iPad - Compare Models - Apple
The current iPad Air, iPad mini and iPad Pro models support USB PD (Power Delivery) Fast Charging. This feature may be significant in your consideration of a replacement model.
Returning to your current iPad, these support pages should help with troubleshooting battery and charging issues:
Charge and monitor the iPad battery - Apple Support
If your iPad won't charge – Apple Support
Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item. Be aware that most models of iPad require a Power Adapter rated at 12W or greater to reliably charge. 5W Power Adapters intended for older models of iPhone are inadequate to charge an iPad.
If you continue to experience difficulties, you might be best advised to seek assistance directly from from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad, Power Adapter and cable(s):
Genius Bar Reservation and Apple Support Options - Apple