My vote goes to Mindcrack.
I like ExtraBees and GregTech, both of which are missing from DW20. I also have no intention to ever use Equivalent Exchange, to me it just looks and feels like cheating.
I do kinda miss Mystcraft, but I've read that it might be added in the future. I don't really care though, Mystcraft mostly makes the game easier.
With both packs I have a list of mods I disable and some mods I would like to see added (or add myself), so neither is a perfect pack for me. Ultimate will no doubt also have mods I don't like and miss mods I do like, but I think it has the biggest chance of including everything I like. If it doesn't add much or mostly mods I don't like I might stick with the Mindcrack pack though.
As for Voxelmodpack, I've never tried or looked into it, so I can't really judge that. I'm having a hard enough time learning about all the other mods already