This is essentially the definitive Captain America guide. It's a nice balance of survivability and DPS. For example, at 4500 Omega Points I am sitting on about 28k health with this build. My TTK is around 45 sec or so. Will be much lower TTK when I reach 7500 Omega.
http://marvelheroes.info/build/7521/Additionally, here's my short list of top tier heroes:
Venom (although he's getting nerfed/fixed on the test center this week)
Scarlet Witch
Hulk
Iceman
Captain America
It is worth investing your Eternity Splinters for any of the above, though I would hesitate buying Venom right now.
Scarlet Witch is the least gear-dependent of all of the top tier heroes, and she's simply amazing. I would suggest starting there if you like spell casters.
If not, Hulk and Captain America are both very solid, but require a little more rare artifacts to reach their full power.
In regards to Doctor Doom, don't give up too early on him. He's got a couple of viable builds as of right now, but there's still nearly a week of iteration and testing to do.
My only problem with him is that his damn Doombots don't have cloaks! Ugh!!!!!! But, honestly, he's gonna be sick.