Hard to say, but the following is cliche but true, 'it's different strokes for different folks'
I'd suggest running the toons you think you'll like together up to say, level 10 and then see how you feel about them. However, this seems slightly less fair on the MM's as they shine more and more as you get more pets!
I still haven't tried some!
Have fun!
"Like War_Hero said, leave it as it is, it toughens up the softies." - Naz Nomad
Union Heroes
War Borg - Leader of The Paragon Knights
Union Villains
McCabe - Co-leader of The Rogue Knights
Defiant - Supreme Hero
Hello again Outkaster. So you like masterminds huh? Goody.
Heres my personal thoughts on each available MM.
Necromancy: Gross!
Robots: Cliche.
Ninjas: Coo O.o ool.
Mercenaries: Coo O.o ool.
Thugs: Matrix. It certainly doesn't get any cooler.
Jokes aside, what exactly do you wish to know buddy.
If you like screaming at your PC screen with phrases like "FFS stop trowning your bloody star and do some F*)!"ING damage" then definately go Ninjas. =D
Bots/Mercs - Ranged attacks - use control powers to keep mobs at a distance.
Ninjas/Undead - Melee attacks - debuff your enemies to reduce thier damage output/dfense and make you pets last longer.
All about personal taste, and what you think is "cool" just like most things in the game really.
Personally, I think big robots are cool, and that mercs and ninjas are cliches (Ghost Recon / Generic Martial Arts movie anyone 
?) Someone else will think that the ability to raise and control the undead is cool and that Thugs are just cliched gangsta rapper chav wannabes.
Best advice has already been given.. try 'em all, see which ones you like playing and that you like the concept of.
@SteelRat; @SteelRat2
"Angelina my love, I'm a genius!"
"Of course you are darling, that's why I married you. Physically, you're rather unattractive"
http://faces.cohtitan.com/profile/SteelRat
Ok so I am new in CoV(never played CoH either) and I am testing the chars... I got an Domi Ice/Cold. I like it, but I like mastermind too...
So I don't know which one should I chose...
Can you give me tips and builds?
thanks