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I am new to 30k, what army should I get?

Imperial fists or ultramarines

Knowing nothing else about you or your preferences I'd suggest Imperial Fists; their Legion rules support a wider variety of builds and their buffs are easier to use.

I am open to any legion, but I prefer some sort of slow, but hard hitting force.

Both have broad buffs that support a variety of playstyles, but the Ultramarines require you to take MSU forces and pay attention to coordinating your activations while the Imperial Fists just give you straight buffs to some pretty basic things (bolter fire, challenges).

Are you more interested in painting 2,000+ points of yellow power armor, or blue power armor?

Most likely blue. I already have a 1500 point 40k Imperial fists army and I wish to see something that isn't yellow.

General advice for starting 30k:

-Don't start with the Primarch. Primarchs are big and shiny and fine to have, but they're also degenerate and uninteresting in small games, so you should try to get to at least 2k without your Primarch.

-Pay attention to your Rites of War. You'll almost always pick a Rite first and build your army around its requirements/buffs second.

-At least consider Pride of the Legion for a Rite. Most 30k armies use 10-20-model bolter or pistol/chainsword Tacticals as Troops; PotL lets you use Tactical Veterans (5-10 models with one special/heavy weapon per five) in Troops, which looks more like a 40k army and is cheaper/faster to get on the table.

Specific advice for Ultramarines:

-MSU. Take small units as much as you can. Your Legion traits (the to-wound reroll and the charge reroll) really key off of having several units doing the same thing in parallel.

-Unique units: Invictarii are the most useful, they have AP2 at Initiative so are great for deathstar fights. They're very expensive, though, so hold them for bigger games. Locutarii are good at bullying weaker units, they can't take a hardcore dedicated melee unit but they'll eat most power-armoured units. Fulmentarii are silly but incredibly expensive.

-Take a backfield HQ. An Armistos, Siege Breaker, or Master of Signal camped with an out of the way shooty unit will avoid duels and avoid handing off the extra VP.

I'd actually advise against MSU for Ultramarines. 30k is designed so that all squads have a 'tax' of 25pts or higher which means bigger squads are effectively paying less for the same number of models. This saving is important, even if Ultramarines get a teamwork bonus.
The better solution IMO is to take a few units as dedicated 'markers' and then keep your infantry squads big. Vehicles are the best solution IMO because they don't lose anything by shooting first, but still count as an Ultra unit for the purpose of marking.
Otherwise the usual advice of taking big squads with artificer sergeants still stands.

In summary, it's a fairly minor (although universally useful) buff that's very easy to trigger. I don't think it's wise to try building your list around triggering it, especially when that build is contrary to conventional 30k list building wisdom.
Rerolling 1s to wound isn't going to make MSU Tacticals better than using Rapiers or Arcus or what have you.
Its a better idea to just write a good Legion list and treat it as a nice little perk rather than something that defines the faction. As opposed to something like WE Chainaxes, which absolutely should influence list building.