Something important that I think should be said in regards to the tier list and how people are ranked and what not. The current tier list, in my opinion, is very very accurate in the top 3 tiers. Below that though, it's a different story. My suggestion was simply pick up a higher tiered char, preferably top or high, to have as a secondary to help fend off your bad matchups. I didn't say to not play low tiers, but to simply pick up some help to go with it.
Also, on low tiers, the main reason that I played Mewtwo for as long as I did is cause he was my best and in order for me at the time to perform at my best I would have to play as Mewtwo. At lower ends of play the tier lists matters less and less. After experimenting with high tiered chars, switching back to mewtwo, then focusing on marth as a secondary, somewhere along the lines I breached into the next stage and my developement, and once that happened I was able to see how Mewtwo is very lacking.
Don't get me wrong, I love to play Mewtwo, and I think he is a "good" character. Potentially mid-tier matierial, which I hope Taj will make into a reality. However, playing as marth, I can already give you a large reason as to why Marth is quite a good bit better than Mewtwo. It's not just in combos and kill moves, and most of what is debated in the tierlists. It's his ability to punish mistakes with INCREDIBLE ease, and the ability to generate fear and control over your opponent by just a random tip or by simply taking advantage of their mistakes. Marth is also able to create an offensive wall that stifles and paralyzes opponents causing confusion and disarray in their play that makes him seemingly unstoppable if one is using Marth with excellent spacing and tech. Mewtwo doesn't have all these strengths. He has a lot of different things to help compensate a little bit, but it's just not as prevalent. I'm not saying it isn't possible to do things like that with Mewtwo, but Marth would be able to do it easier, better, and more often.
Trust me, I have spent a lot of time thinking about this. I really love Mewtwo, and I hope to god he is incredible in Brawl, but in Melee I feel now, more than I have ever, he is just too lacking and would be a hinderance to me if I spent my time focused on improving him instead of Marth. That said, Mewtwo is still my secondary, my oddball counterpick character. Mewtwo has a lot of merits that could help me win in conjunction with Marth, and I realize that.
I hope this makes sense. Also, I would still like to make one last Mewtwo combo video, if I could ever get my video back from a certain Memphis/Millington crew *nudge*.