I can only afford 2 battle packs right now so wanted to know what is the best value for my money. All I own is the core so mostly anything will be helpful. The races I like are dwarfs and skaven (skaven because that is what I play on the tabletop game).

Are you just playing for fun, or competitively? For competitive play, the Skaven are much better than Dwarves right now. For casual play you can certainly play Dwarf decks that are pretty decent though.

The best Skaven are spread out throughout 3 different battlepacks. Clan Moulder's Elite is in Tooth and Claw, Clan Rats and Greyseer Thanquol are in the Skavenblight Threat and Deathmaster Sniktch is in the Deathmaster's Dance. Greyseer, Elite and Deathmaster are all just 1x in each BP, so you have a problem if you want to field a playset of Skaven.

In terms of building the best possible deck, here's what I would do. Buy 1x Skavenblight Threat, which gives you Clan Rats, 3x Spider Riders and 1x Greyseer. All of those are great cards. Then get Tooth and Claw for Elites, Gutter Runners and Rat Ogres.

With just those and 1x Core Set and assuming you can't borrow cards from a friend, this is probably close to your best deck:

3x Spider Riders
3x Crooked Teef Goblins
3x Gutter Runners
3x Clan Rats
1x Clan Moulder's Elite
3x Squig Herders
2x Followers of Mork
3x Festering Nurglings
1x Greyseer Thanquol
1x Big 'Uns
1x Rat Ogres
2x Boar Boyz
1x Ironclaw's Horde
27 units

3x Warpstone Excavation
3x Contested Village
1x Orc/Chaos Alliance
1x Chaos/Dark Elf Alliance
2x Armoury
1x Rock Lobber
2x Choppa
1x Totem of Gork
14 supports

1x Innovation
3x Seduced by Darkness
1x Waaagh!
1x Pillage
2x We'z Bigga!
1x Troll Vomit
9 tactics

You will lose to people with Deathmasters. A Dwarf deck will have the same problem, but at least this deck kills faster so they have less time to draw one.