With my friends, I play whichever characters I find fun: in Brawl, I found characters like Olimar and Marth (and sometimes Fox) to make things seem too easy (and less fun as a result), which may indeed have to do with them being high tier... (and Olimar's up smash... it's dumb how strong it is.) Well, high-tier, except for Fox... I like them as characters, to play as and otherwise, and still use them in more serious matches, but I tend to avoid them for the majority of my play.
On the opposite end of the spectrum, I found characters like Yoshi and Link really underwhelming, especially coming from Melee where I played Yoshi all the time. While certainly challenging enough to play as, I guess they just weren't interesting enough, potentially because they felt weak, Link in particular: this, again, lines up with the tier lists of the game, though it was based on my own experience.
So who did I find most fun? I played Sonic too much, since he's my favorite, but he's not my most-used character... no, that would be Ike! Both Sonic and Ike are the characters I find most fun to play in Brawl. They have strengths that are fun to use to win with, and weaknesses that are enough to feel like fights are fair (perhaps because I was almost always at a higher skill level than my friends).
Interestingly enough, if you look at the tier list, Sonic and Ike are just about the most balanced characters in the game: by this I mean that they don't have any hard counter-pick characters and aren't a hard counter-pick character to another character. They have worse matchups against higher-tiered characters and better ones against lower tiers... in general, they don't overpower or get overpowered by anyone too badly. They can hold their own, even if, in the truth about tourney play, they just aren't good enough to cut it.
So, what to take away from this?
Personally, I found higher tier characters in Brawl to be less fun to play as with my friends, since they felt a bit overpowered, regardless of whether I had seen a tier list or not. But I also found low and mid-tier characters not so great, either. What really did it for me was a character that I liked a lot, and I felt like they had strengths that were fun to use. That's it.
As far as Smash 4, I'm extremely excited to see that people are saying that Fox and Sonic are good, probably B tier or so... and Yoshi! I basically have no words... he looks simply amazing, even before I saw anyone saying he may be top-tier! Some of my favorite characters having the tools needed to be competitive makes me really happy, and I look forward to using them to destroy more serious opponents. FOR GLORY!
This means, though, that, depending on how strong they really are, I may gravitate towards lower tiers for friendly matches, and avoid my favorites. Ike still apparently isn't so good, but I bet I won't agree so much, and will use him a lot. Shulk looks extremely fun, Captain Falcon looks glorious, and Marth... we'll see about Marth.
But really: it's the cliche thing to say, but play who you have fun as. If you find yourself having less fun, try another character for a while. You can always go back. Or, you can search for stronger opponents, in which case you should have no worries about whether your character is overpowered. Don't worry about tiers unless you're looking at winning a serious competitive tourney.