Death Guard, Iron Hands or Salamanders are probably the army for you. I'm going to assume you're bringing the Legion-specific Rite of War for maximum flavour.
Death Guard have powerful templates (all Flamers can get Shred and Gets Hot for free), can bring the 30k equivalent of Devastators as Troops, have Move Through Cover, and Independant Characters can buy fricking Rad Grenades for 10pts which makes them scarier in assault than you'd imagine from an army of Devastators. However, you can't Run or Flat Out, and you can only bring one Fast Attack choice which can get you outmaneuvered, especially by more mobile Legions like Emperor's Children, Sons of Horus or Night Lords. Mortarion is tough as hell (T7 W7 with 2+/4++ and re-rolling his IWND) and gives all Frag Grenades, Frag Missiles and Havoc Launchers in your army Poison 4+ which for Missile Launchers is usually going to be a re-roll To Wound. He's got a pretty strong Str8 AP2 Instant Death Unwieldy scythe and a cool anti-tank pistol. He's very resilient but Plasmas and Meltas can strip wounds off of him pretty quickly.
Iron Hands have a super powerful Legions Astartes rule; -1 Strength to all guns fired at them. They'll win any Legion vs Legion shoot-out presuming equal forces other than different Legion rules. However, they get a bunch of restrictions to compensate for that very powerful benefit; they have to take a Ld check before Running or Sweeping Advancing (SA is a big deal since nobody in 30k has ATSKNF), and can't Go To Ground. They also can't have more Jump Infantry, Bikes and Jetbikes than they have Infantry units, so if you had your heart set on a Jetbike army, they're not for you. Rite of War gives them Stubborn in their own deployment zone, anyone who can take a Flamer can take a Graviton Gun instead (not the same as a 40k Grav-Gun; a Haywire gun that makes you take a Str test or take a wound allowing saves, so mainly an anti-tank gun that can do anti-infantry in a pinch), Tanks get Outflank, and you can bring Mechanicus robots in Elites. Downsides are only one Fast Attack choice, and only one Consul (think specialist HQs - Librarians, Chaplains etc. but there are more types in 30k - other than the Techmarine equivalent). Ferrus Manus is a pretty cool guy; gives all vehicles with at least one AV13+ facing IWND, is very tough against shooting because of the -1 strength of shots against him and his 2+/3++, and has more guns than most Primarchs. Has an AP1 Thunder Hammer that doesn't have Unwieldy.
Salamanders are also slow and tough; not as tough as Iron Hands, but braver. They re-roll a single D6 on failed morale checks, auto-pass Fear (suck it, Night Lords!), have +1 Strength on their Flamers (and with their unique units they can take a SHITLOAD of Flamers) and enemy Flamers have -1 Strength, but they don't add their Initiative to their Sweeping Advances (so you basically can't Sweep, which sucks) and you have -1" on Run and Charge moves which can be a pain when trying to get onto objectives or charge. Can't bring Moritats (think John Woo as a Space Marine suicide gunner) or Destroyer Squads (Marines with dual pistols who like giving people cancer). Rite of War gives their vehicles 5++ saves against Melta, Flamer, Plasma and Volkite making them a fair bit tougher, Multi-Meltas/Meltaguns/Melta Pistols get Master-Crafted, you can bring some pyromaniac dual-wielding flamer Marines as troops, and you get Move Through Cover but you can't Deep Strike, you gotta have more Troops than Heavy Support and Fast Attack combined (can be a big problem trying to fit ranged anti-tank in your list, but when you can spam Master-Crafted Melta in Troops it's not as big a deal as it is for, say, Night Lords). Characters are tough because they can all bring Eternal Warrior (almost exclusive to Salamanders other than on Special Characters in 30k), and can bring special Storm Shields that are a 5++ or +1 Invuln if you already have one (so Cataphractii Terminators, which soon everyone in the world will have because they come in the Betrayal at Calth set, will go to 3++). You can make people hate you in a front-line fight because you build units of 2-wound Terminators with 2+/3++/Feel No Pain. Vulkan is super tough (halves the strength rounding down of any Plasmas/Flamers/Meltas/Volkites shot at him, which makes him straight-up immune to Plasmas and Volkite), gives all Salamanders +1 Leadership (making them the second-bravest Legion after Word Bearers), and royally fucks up hordes which is unusual for a Primarch who are usually better at killing non-Primarch characters and elite units.