Well it is often considered that Rotom-W is the best form to run on Rotom due to the presence of high reliability. Though all the Rotom forms are specify in one great aspect, Rotom-W seems to be good in all as it has great Special Attack, decent Speed, Great Defense and great Special defense. Though it's typing should be what makes him so good because he practically only has one weakness and that would be to grass since he has the ability levitate.
He can often be aided by Politoed and can take all the other forms with relative ease, including Rotom-Mow
As for the Deoxys form, this is a little harder as they all of have monstrous stats depending on their typing, however the one that you will be finding the most useful should be Deoxys-D due since it can lay entry hazards with so much ease and usually grant it's teammates the victory as we all know that entry hazards are very annoying. Out of the 3 he will also survive the longest. However if you were to go offensive, Deozys-A should be superior as it has massive speed and scary offensive abilities, however it is very frail meaning it will kill everthing until it is hit by a move
- Rotom-W is the best form for Rotom
- Deoxys-D is the best form for Deoxys shortly followed by Deoxys-A (depends on the role you seek)