Hmm... Well, since you're not worried about sticking to any particular tier, here are my opinions for what they're worth:
Arcanine: Leftovers is kind of a waste here. Though they have decent HP and defense, they're probably not going to stick around for a really long time so they won't get much benefit from healing items like Leftovers. That Fire typing does them in pretty darn quickly, since there are so many water, ground, and rock moves out there. You'd likely be better off giving Arcanine a Gem, a Life Orb, or a Choice Band to max out its attacking/speed capabilities. Arcanine usually does best when it's allowed to hit like a truck.
Yanmega/Ninjask: Well, Yanmega can't learn Baton Pass or Swords Dance at all. If you're set on that moveset, you'd have to go with a Ninjask. Which could be cool, except that Ninjask are pretty fragile and everyone knows what they're going to do when they come out. What's more, it usually only takes one or two moves to shut down Ninjask, be it through Taunt, Haze, Mean Look, or simply beating the poor bug to death. Unaware is another thing to really watch out for, since it means you're just wasting turns. Don't get me wrong, a SD-passing Ninjask can be really effective if you are able to take advantage of an opening the opponent gets you. Just, beware that it can be very tricky to pull off.
If you do end up deciding to go with Yanmega instead, you won't be able to really use it as a setup poke very well. However, they've got great Special Attack, good Speed (and a beautiful ability in Speed Boost, meaning you don't have to invest EVs in it if you really don't want to), and pretty good HP which can offset their frailty a bit. So you could make one into a pretty awesome Special Sweeper. A set based around Bug Buzz and Air Slash allows you to take advantage of both STABs with specially-based moves, they have access to Night Slash, Psychic, and Shadow Ball if you need to have some extra coverage. They get Tailwind by move Tutor in Gen IV (and in B2/W2, if you want to plan ahead a bit) if you want to use it to set that up. Thanks to the Dream World, it is possible to get one with Giga Drain too (it will be possible to tutor this move too, when B2/W2 are out), which can be another interesting special attack. Substitute can extend your life a bit (especially if used with Giga Drain) and Whirlwind can phaze out nastier opponents if you wish. There's quite a few possibilities for a Yanmega! Just make sure you don't send it out into Stealth Rocks. You'll lose half your health. Ouch!
Donphan: First off, I'd recommend not bothering putting EVs into Defense. That stat can take care of itself. You'd probably be better off splitting those EVs between HP and Sp.Def. Generally speaking, it's better to invest in HP rather than other defensive stats. HP increases your endurance regardless of whether physical or special attacks are used against you, so its like boosting up 3 stats in one. Kind of. Sort of. You really only want to invest in Defense or Special Defense if they're absolutely abysmal and they totally need to be boosted a bit. Donphan is actually a good case of this, it has great Defense that needs no extra help and good HP, but it needs some Sp.Def. So, I'd just suggest swapping those Defense EVs for HP EVs. You're most likely to end up with a longer-lived Donphan, especially since its greatest weaknesses are mostly specially-based (many Water, Grass and Ice moves are special.)
As far as moveset goes, I'd suggest swapping out Rollout for Stone Edge. Rollout is a really risky move to pull off competitively, because it makes prediction a breeze for your opponent; you're locked into that move for 5 turns (unless it misses), doing pretty light damage for 2 of those 5 turns. Most human players in most common circumstances will be able to take advantage of that and turn the battle in their favour. Besides that, locking up a Spinner doesn't seem to be the wisest choice. Of course, I have very limited experience with spinners (I usually just absorb hazard damage and get on with life, lol!), but I don't think I'd want my spinner to be locked into Rollout if I had something like a Volcarona, Charizard, or Yanmega in my team and the opponent set up hazards... At any rate, Stone Edge has 80 accuracy compared to Rollout's 90, but you're doing a consistent 100 damage and you don't have to be locked into anything.
For your last moveslot, well, I'm not too sure what to recommend. There's a lot of different moves you can use. Poison Jab or Body Slam (Body Slam must be bred onto it) are useful because they have a decent base power and can spread some status. I'm kind of leaning towards a move like Assurance, however, because it's one of the only good Dark type moves it can use, and you'll have to be wary of spinblockers. Something that can smack away a spinblocker would be very useful for your Donphan
Milotic: This is probably where you'll want those Leftovers that Arcanine is holding. Milotic get Marvel Scale as their normal ability (Ups Defense if they have a status ailment) and Cute Charm as their Dream World ability. A Milotic I have personally used had the moveset of Aqua Ring, Recover, Ice Beam, and Scald. It was a pretty straightforward moveset: Tank as many hits as possible and keep dealing hits that have a fairly good chance of statusing the opponent. My Milotic also was very good at stalling out opponents that she couldn't really hit and that couldn't hit her (like Vaporeon), since Aqua ring has such a high PP. If you really want another attack though, Dragon Pulse might be a nice choice. Only Steel Type resists it, but that is neutrally damaged by Water anyway. The only real problem would be something like Magnezone which resists water, ice, and dragon, but you don't see those too often anyway. As far as EVs, again, I'd suggest swapping out those Sp.Def EVs for HP EVs. Defense is kind of low, so you would probably benefit from a bit of investment there.
Or if you wanted to think about something new, a really common set for Milotic that takes advantage of their Marvel Scale is Rest, Sleep Talk, Dragon Tail, and some other attacking move. The beauty of that set is that Sleep Talk allows Milotic to use Dragon Tail without DT's negative priority. Which can be really great if you're up against a slower opponent.
At any rate, I hope that at least helps you get some ideas for your pokes. Good luck and have fun constructing your team!
