In 6mm Irregular has a decent range in a range of calibres, although you sometimes need to scavenge the various ranges - the naval gun on a land mounting (colonial range?) usually produces oohs & aaahs of appreciation when it appears on the tabletop, which in East Africa etc it does with astonishing regularity!

In 15s field guns are very well represented, and the heavies are covered by the Really Useful Gun range, now cast up by Irregular as well.

In 20s Emhar do very, very cheap plastic 18pdrs & 77s, plus some of the metals manufacturers do bigger stuff (IT, Tumbling Dice?). The RUG guns will also do for 20mm.

Being deeply sad I've got WW1 armies in 6mm, 15mm & 20mm, but all the heavy stuff is either 6mm or 15mm, which is fine when using the 20s as well as the guns are so far away they look smaller anyway;) Depending what sort of games you are thinking of doing you don't need masses of heavy artillery as most of it will be parked some way behind the front anyway, however it is nice to have some around if playing especially megalomaniac (ie multi-Corps) battles or deep breakthrough situations.

If starting from scratch in WW1 I'd go with 15s, cheap, huge ranges, easily available and still big enough that you can see what they are supposed to be.