What is the best tier 5 arty in World of Tanks

It is completely down to personal preference and playing style..

If you want long range, hard hitting (up to 1000hp) and 56kph top speed you want the M41.. However, with second gun I can not even get 3 shots a minute.. But IMO this is the best tier 5 arty.

For fast rate of fire and OK TD mode you want the Hummel.. But there are two drawbacks. Firstly is is the least powerful of all tier 5 arties and also, the ammo is absurdly expensive. Up to 1100 credits for one shell.

SU-8 is an average tank.. Very similar to the Hummel but harder hitting and less expensive ammo.. I wouldn't recommend going Russian for arties as they are extremely outclassed in the higher tiers. Except SU-26 which is plain epic.
Finally, the French. The AMX is an okay tank in general. For a tank. Not an arty.. The accuracy is bad, reload bad, power terrible but it is quite fast. I would not recommend this route..

So.. It all depends on playing style but after years of research (joking, about 2wks) I have concluded that the arts go in this order:

1. M41
2. Hummel
3. SU-8
4. AMX

Thank you. I hope this helps you in your WoT career.