Sweden is great, with a very good starting position and a lot of room for expansion, since you start either at war or uneasy with all neighboring factions yet you can usually secure an early alliance with Prussia, who regularly becomes a powerhouse.
Great Britain is also a great choice, since they are pretty safe on their European island nation and can expand greatly in the Americas early on.
Russia, like Sweden, has a pretty secure starting position, but they have poor quality units so won't be the easiest campaign.
Conversely, Prussia has a less than outstanding starting position but have the highest quality units (at least early on, anyway) and can outclass their rivals.
Lastly the Marathas have an easy going early on against the Mughals, but be wary of Mysore, the little red nation in the middle of their land, as they seem to always be traitors.
IMO, these are the best starting nations, and since a strong early game usually carries an easier going later on you should be fine with any of these choices.
The Marathas have access to immense amounts of trading goods and can get extremely rich very quick if you secure enough trade routes. It's also a good idea to research naval techs with them so you can protect your ports and routes against raiding.
I agree with z129000's choices, with the exception of Prussia - the difficulty of their starting position cannot be understated. It's almost a guarantee (especially for a first-time player) that you'll be at war with two major powers within the first couple turns, and probably a minor faction or two as well. Depending on how much the AI decides to pick on you, you're starting with an uphill battle. If you're looking to jump right into the deep end, go ahead, but if you want to ease yourself in, Great Britain is practically made for beginners. Good units, a practically invincible homeland, a lot of starting ports, and some of the better colonial holdings all make it a very newbie-friendly start, but the real reason I recommend them is that it gives you a goal. Great Britain, unlike most factions, starts the game with a mission to capture several French and Indian territories in North America, and as a reward the 13 colonies will become part of England. It gives your campaign direction at the start, so you don't need to worry as much about planning your moves while you learn the ropes. It gives you a bit of a chance to ease in before you have to start making the big calls all on your own.
Great thanks for the feedback, I just got the game last week, and I've played ME2TW, so I have some familiarity with the genre, but I've found that this game is much more involved than previous installments. I've just started a grand campaign with Britian and I am currently building up my wealth so I can get some armies together and start expanding!