I think it's a tie between DW20 and Mindcrack. Mindcrack is close, but a bit harder. DW20 is also close, but a bit easier. I'm pretty sure both have more mods than the Beta pack used to have, but some of the old ones aren't included (For example Mystcraft in Mindcrack or GregTech in DW20).
It's kinda strange though as it sounds like you're asking for an opinion while it's really just facts you're after... You can also wait till the Ultimate pack is released, it probably has everything the beta pack has and more (though they haven't announced exactly what mods will be in there).
Finally, there is a sticky topic (here:
http://forum.feed-the-beast.com/threads/different-ftb-packs.4727/) which shows the differences between the new packs. If you pull up a list of mods in the Beta pack (for example in the launcher) you can easily compare yourself and see what you think is closest (which would depend on how you prioritize the different mods, not how close they really are if you just count mods).
