I actually had quite a challenge starting off as Spain.

Although you start off big, you also have a lot of issues.

As I pointed out in the guide, your king is basically a total dead weight. Not only does he not give you benefits, he gives you -10 to all relations!

All that territory is a bit of a dead weight, too. Unlike Russia, you cant just defend certain points, because, starting out, almost everybody hates you. It's a pain to develop those outlying American provinces as well, because your initial income is a bit small and the pirate fleets will happily take on your ships and win.

Now, it is true, you do start off with some advantages. The, uh, two people who DONT hate your guts, France and New Spain, will probably shield you from any serious early threats. You might lose Florida and Portugal might run amok in Spain, but you probably wont get taken over.

I wouldn't say Spain's the HARDEST faction, but the required military buildup early in the game can hurt your early growth a bit. I'd rate them as challenging.

Easiest, IMO, is Russia. Great starting position, nobody really hates you besides the Ottomans, and you've got a decent monarch. Plus, your've got good starting infantry options. Cossacks and Stretsy make an excellent team. Taking the Balkans from the Ottomans is, if you will forgive the obvious metaphor, like taking candy from a baby.

Hardest, I think, would be Dageistan :P