Dialga
If i would happen to get good IV; Should get Steel or Dragon type moves?
Answers
If you're planning to use it as an attacker for gyms/raids, Dialga with steel moves is beat out by Metagross (with Meteor Mash), and Dialga with dragon moves is beat out by Rayquaza, Dragonite, Palkia, etc. This is, of course, just looking at DPS. Dialga will have higher TDO than other dragons in some instances due to it's steel-typing giving it a bunch of resistances. So I would go with dragon moves if you want a good attacker that has some bulk behind it.
I think Dialga's biggest niche will be in PvP. Dragon breath is an incredibly strong move, Iron Head comes out fairly quick and Draco Meteor pretty much one-shots anything that doesn't resist it. It only fears fighting and ground-types, so if you're using it in the Master League, it really only fears Groudon (as fighting-types don't see a ton of use at that league).
Dragon. In its absolutely fantastic against dragon types like palkia and the lati's, and after playing around on pokebattler, theres some cases where it can outperform rayquaza because it lasts significantly longer because it can stand up to dragon moves. (Looking at pokebatler V palkia with DM, it outperforms dragonite, salamence and palkia reliably, and aginst dragon breath DM, is superior TTW to rayquaza, and againt TD, only 3 seconds behind). In a trio, the diference between time an all Ray team and an all dialga team is one second, but dialga faints only 32 times to ray's 51. Looking at other dragon legendary match ups, Ray out TTW's dialga, but dialga always faints out much less.
Steel is locked down extremely tight by MM metagross
"Once again cherry picking raid bosses and move sets to support half assed opinions..."
Umm, that's kinda how it works when you're talking about how Mons perform as raid attackers, because those are the scenarios you'd use them in. What, were you expecting him to mention Dialga's usefulness in a Machamp raid (where it's horrible) as a point of reference as well?
Are you capable of contributing anything beyond negative/idiotic commentary? Because there's nothing wrong with MetagrossMaxis's post.
I'd say in PvE there are scenarios where one is a better choice over the other because of the resistance profile and the overall better performance of Dragon Tail over Dragon Breath. For example against a Fire/Dragon mon like Reshiram with Fire moves (to put a likely future raid boss) the clear winner is Palkia, but against an Electric/Dragon mon like Zekrom with Electric moves (it's Legendary counterpart in Gen5), Dialga would be a better choice.
However, even in those artificially picked examples: Rayquaza or Dragonite will be much better counters thanks to DT/O, and then there's Garchomp playing in a completely different league against them.
