All Heavy Artillery SFs Ranked

S Tier: Katyusha

Deals a lot of area damage, moves fast and kills gens easily. Top Tier.

A+ Tier: MK F3

Karl but better. Destroying forts sucks though. Also, artillery takes the first hit, not the last.

A Tier: Elefant

Hits tanks really hard. Works great on Eastern Front and Further. But same as MK F3, artillery hits first.

A- Tier: Karl
Probably has 3 speed when maxed and good scout. Abilities are inferior to Elefant though.

B+ Tier: Transports

S P E E D. Works great when your non-main SFs are used up (like in star campaigns or elite missions) or when you are short on cash.

B Tier: M12-GMC

C R I T S. Like the Maus. Jungle mobility is cool too, but kinda contradicts its first ability. Transports are better if this isn't maxed.

B- Tier: Maresal

Only use when you need to kill an arty gen. Transports are better than Maresal if it's not maxed (and who will max this anyways?)

Moral of the Story: Katyusha + Elefant supremacy. Karl and Transports second.

Edit: My NEW ranking

S Tier: Katyusha

Area damage still very useful.

S- Tier: MK F3

1 extra range is good, but right now I don't really see when exactly I would need it. I believe it is better than the Katyusha, but it is an IAP, sooo...

S- Tier: Elefant

Movement, damage and gen-cracking all in one. Leeb actually makes the attack again perk useful.

A Tier: Karl

In United Front, best F2P on this list. But otherwise, in reserve.

A Tier: M12-GMC

Good damage, 3 movement when maxed, crits... Pretty great. I think MK F3 should have M12-GMC's first ability. 

B+ Tier: Maresal

For moving through mountains with Leeb and killing arty gens.

B Tier: Transports

No need to max. Cheaper than all the rest. When you run out of SFs, whatever will you do?