Well, again, the question is what other keys you need to press at the same time, and what the rest of your keyboarding context is. Personally, my preference is to not use a standard-layout keyboard at all, but something with column-staggered keys and a more natural hand shape. Then I put the arrows on a layer on the home row positions for the thumb and three fingers on one hand.
If you have a standard layout keyboard and you often need to use random keyboards you don’t own (so keeping normal arrow layout muscle memory is helpful), and you need to alternate arrows with typing, then using something like an inverted T style layout with IJKL representing up/left/down/right (on a layer where the layer switcher key is on the left hand) isn’t too bad.
The advantage of using the thumb and index finger for up/down and the middle and ring finger for left/right is that you don’t need to overload one finger with performing two different functions, and you can also make sure that any diagonal directions are convenient to press and to quickly alternate back and forth. Alternately pressing two keys on the middle finger / ring finger like 343434343 is pretty uncomfortable and slow. Including an index finger or thumb in the alternation is much better. That is, 232323232, 242424242, 131313131, 141414141 are all much more comfortable than 343434343. [Where the number 1 = thumb up through 5 = pinky]