What? You all honestly think Kakashi could solo Gaara?? Be realistic. Kakashi wouldn't be able to get close to Gaara. As a matter of fact, he MIGHT be able to solo Mei with high diff, possibly Tsunade too, if he Kamui'd her. He's powerful/intelligent and I would say he's Kage level, but you're taking it too far w/ Gaara.
-Well I don't think I'm taking it that far personally. To defeat Gaara, Kakashi wouldn't really need to get close to Gaara to defeat him. Kakashi has Water, Lightning and Earth techs that he can use as distance attacks. Not only that but adding in Kamui(which I hate adding in) is a real booster. Kakashi is able to use Kamui on full bodies without it really being noticed. Just like when he Kamui'd a Naruto clone without Tobi realizing it.
--Now I'm not saying this is a walk in the park for Kakashi. I know how powerful Gaara is and I know if Kakashi and him were to fight ether one can win but I feel like Kakashi has a higher chance.
Know Kakashi vs Mei, I think Kakashi has this. From what we have seen we know more about Kakashi and his battle tactics then Mei. IMO Kakashi can easily out think her and end up winning this fight. As for Tsunade, seeing as she is mostly Taijutsu and slower than Kakashi, he could possibly keep his distance and use ninjutsu or a lightning clone. The best Tsuande can do is prolong the fight and make i harder for Kakashi.
These are just my personal opinions of course though.