I'm going to say Necrons for a few reasons.
1. You already have Iron Hands, and running another Space Marine Chapter seems redundant.
2. I know nothing about Tyranids.
3. NOTHING is as durable and resilient as Necrons.
To address other things:
Dark Angels can NOT play entirely Deathwing. It's baked into the rules for their formations: the Deathwing have to start in reserve, which means an entire army of terminators will lose turn 1 guaranteed. You can offset this by adding Dreadnoughts in Drop Pods, but remember that only half of them (or 1, if you only take 1) will drop first turn, which means you still have a chance, if you go first, of being tabled on turn 1 by your opponent. If you really want all terminators, and like tanks, I'd recommend a Landraider Spearhead (I wouldn't be surprised if you had some from your Iron Hands) and then Deathwing.
If you want all Terminators, you can also use Grey Knights, who legitimately have Terminators as a Troop choice. They lack heavy ranged firepower, however.
Necrons.... These guys are pretty impressive. In a standard, normal Combine Arms Detachment, most of the army gets a 5+ Reanimation roll, which is *like* an extra save on top of the 4+ or 3+ that most of the army has, making them more durable than most Marines. If you run the Decurion (which is their version of a Gladius from their codex) that Reanimation becomes 4++. If you have a Cryptek in the squad, it also has a 4++. If you use the Mephrit Dynasty Detachment (from Shield of Baal), it rerolls 1s for that roll instead. VERY durable.
Necron standard weaponry are all full of shenanigans. Gauss weaponry auto glance vehicles on 6s, regardless of strength vs armor. Also auto-wounds on 6s, regardless of strength vs toughness. This comes STANDARD on the basic warrior, with a Bolter statline.
Tesla weaponry is available for options for most vehicles and Immortals (better than Warriors). Any roll of a 6 generate two additional hits. Doesn't affect Snap Shots. Makes them GREAT for wiping out hordes. They have no AP, but they generate lots and lots of wounds, and your opponent has to fail a save eventually.
Necrons have lots of easy access to fliers, because their basic transport is a flier with decent weaponry (S7 Tesla weapons), which even can be used as anti-air also. Their tanks are weird, in most are AV11, but they have special shielding that make them count as AV13 until someone penetrates and 'pops the bubble'. OR they're monoliths, which are AV14 all the time.
They have options that feel Terminator'y in Lychguard, with giant axes or swords and stormshields. They've got jump pack units. They have Triarch Stalkers, which are my favorite, which are like dreadnoughts on steroids: AV11/13 open top walkers that can use a variety of weapons, including a melta with a flamer option also. Good stuff. Their lords are very customizable, some of their special characters have some cool toys, and if you really want to play with the dice gods, there are C'tan, which are the ultimate in random powers.
Only downside (to me, at least)? You building skeletons. Lots of skeletons. It's Tomb Kings in Space.