Garden Party Tote for sure! I lug mine to the park, Target, etc. Great for those days when I don't feel like dressing up. I think the medium sized one was around $1,700? The only thing about the GPT is that there is no shoulder strap for the larger sizes--(wait, I think GT mentioned that there may be a new one with a strap? you may want to verify.). There is also the Caravanne Horizontal--leather, lots of outside pockets with a shoulder strap--I think this starts in the upper $2Ks?? To organize things a bit, I'd recommend getting a Chameleon insert in the GPT because there are no interior pockets.
The other alternative is the Evelyne, but that wouldn't really be too suitable for a diaper bag--even in the largest size, due to its narrow silhouette. Sippy cups, extra cloths, etc. would be bulging. At best, good for some quick runs to the store with one or two diapers and wipes.