1. Blue: One aspect of blue that I haven't seen mentioned very much is Evasion. I love the number of blue creatures with evasion abilities that allow me to go around my opponents' blockers while I sit behind walls. Control is my favourite type of deck to play, and no colour is better at it than blue.
2. White: Gaining life,
Arrest,
Pacifism, cards like
Serra Angel,
Thalia's Lieutenant... I feel like white is the colour best suited to be played by itself (though Blue and Green work fairly well too). White's creatures work together very effectively, and you have access to Exiling, some Evasion, and a good amount of Control. The reason I prefer blue is that white lacks card-drawing and its creatures tend to be slightly smaller than blue's.
3. Black: Black is the colour of death; everyone thinks of removal when they think of black. I enjoy black because I can eliminate most creatures on the board, though anything with Hexproof becomes very problematic. The reason I don't like black as much as White and Blue is that Black doesn't have as many strong finishers (aside from Demons) and that black cards tend to hurt you, which is very risky against Aggro/Burn decks.
4. Red: I feel like Red shouldn't really be 4th, since it's also a very strong colour. The main problem with Red is that it doesn't work as well without support from another color, except in an aggro-type deck. Red doesn't involve as much strategy and thinking as other colours, and often times you can be decimated by a couple of removal spells. Red is a strong colour, but it needs the support of something like Blue or Black to be more effective, in my opinion.
5. Green: I've never been fond of Green. Green, to me, is about getting a big creature out as early as possible through ramping and then destroying the opponent with it. The problem with Green is that it doesn't play fast enough, lacks any sort of removal, and its creatures are very vulnerable to kill spells. Green is also extremely vulnerable to Flying decks unless you go out of your way to get Spiders, etc. There are plenty of strong cards in Green and plenty of strong multicolored cards including Green, but Green to me doesn't seem as powerful as the other colours.