Good Ultra League Team?

I wouldn't recommend venasaur, its poison type doesnt add a whole lot, and doesnt make it as effective of a coutner to ground types. For grass, Ive heard amazing things about sceptile in PvP, fury cutter and leafblade are a lethal combo with how often it goes off, and a second move of either aerial ace or earthquake can cover its counters. Venusaur is also not resistant to ground moves, which I have heard are prevalent, or at least seen often enough to be a threat.

Heatran, I dont know, its double weakness is a liability and theres a good number of ground types, or at least pokemon with ground moves in ultra, same for water types that also resist both its STAB, swampert says hello. I haven't heard many good things about fire on offense for ultra, or even in PvP in general due to the prevalence of their weaknesses. Fairy, its a nonissue from what ive heard, they are slow to charge and no fast move leaves them vulnerable. Ice is a threat from what I have heard, but either legacy or non Tyranitar can deal with them well by packing either stone edge or fireblast paired with crunch (haven't heard almost anything about fighting types in PvP, especially ultra because of giratina, closest would be Lucario, but thats with bulletpunch/shadow ball, and its weak to fireblast)

If you are to have a steel type, i'd recommend Steelix, bulky, resists a lot, and hits giratina for super effective damage, only think it really needs to worry about are water types or Lucario with close combat, but sceptile deals with those well.

Giratina, of course, you cant go wrong.

This is similar to the team I use, though I try to save Giratina for the end when my opponent has run out of shields. Personally I gave my Giratina Shadow Sneak as a second move so it doesn't get completely walled by Metagross and other steel types, though I may change it depending on how my team composition changes.

Having not used Heatran and referring to the GamePress guide about it, it does better as a rock/steel attacker than a fire one due to Fire Blast having such a high energy requirement. I've used Charizard here with Blast Burn/Dragon Claw for fire type offense but your team would likely struggle with powerful ice types otherwise since Charizard doesn't outright resist them.

Give Venusaur Sludge Bomb as a second move. People I play against see mine and expect Frenzy Plant. Sludge Bomb can be an unpleasant surprise for them.

The biggest hole I can see in your team is a lack of Giratina counter other than Giratina itself. If you can force your opponent's out first then you should be able to topple it, otherwise it'll come down to who has the shield advantage.

It seems solid. Heatran is a good switch in a lot of Giratina counters by virtue of its typing, even if its movesets leave a lot to be desired.

Venusaur I might personally rather replace with Meganium, Venusaur is good for grass mirrors but both of your other mons are already strong against grass so the ability to threaten opposing steel-types for example with a late game EQ is I think more valuable in this team. And Meganium has a notably better TDO in neutral matchups as well, letting you win some Giratina counters like Umbreon with a bigger margin.

This may just be me lacking in creativity or buying too much into the "giratina is king of ultra league" hype, but I find that giratina and bite/ice beam/hydro cannon blastoise are must haves on any ultra league team, given that blastoise is the only "good" giratina counter besides giratina itself.

To complement those two I tend to be a fan of metagross (as he handles ice/fairy types well, and blastoise already covers his weaknesses to ground/fire/ghost).

If you're set on using heatran, I might opt for skeptile/meganium over venusaur. That poison subtype isnt super relevant, especially when heatran already covers fairies.

Assuming the TDO numbers are correct, I would take a Meganium with Frenzy Plant & Earthquake instead of the Venusaur. Sure, it's less DPS, but much more bulk and better coverage against Electric & Steel types.

And though it's a ways off, eventually Swampert with Mud Shot & Hydro Cannon is going to be very nice in the Ultra tier as well. Just have to watch out for Grass types with that one.