Mario/Bowser/Geno:What can I say about this team? Well, you're definitely covered on attack. Magic's okay, although Mario will be doing the bulk of the magic, possibly a few magic attacks from Geno. Healing, however, is a Very Large Problem. You don't even have HP Rain, so you better either win the battles quick enough so you don't have to worry about healing, or pack plenty of items.
Mario/Bowser/Mallow:Now, for this team. With Mario and Bowser, attack's pretty good. Your magic is also good- you have Mario and Mallow supplying the four element types of magic attacks- namely Jump, Fire, Thunder, and Ice. Healing's weaker, though- you must rely on HP Rain alone. If Mallow croaks (sorry, bad pun), you need items.
Mario/Bowser/Toadstool:This is the team everyone recommends, including Nintendo Powder. Attack is good- both Mario and Bowser are hard-hitting. Magic is not so good- Mario's the only one here with decent attacking magic- Toadstool's is healing and status ailments which don't work against bosses, and Bowser's magic is fantastically expensive. Healing, of course, is covered by Toadstool. Here, you can put the Safety Ring and Lazy Shell on her- with Mario and Bowser around, the defensive power is more important than attack.
Mario/Geno/Mallow:This is the team you must work with from Forest Maze to Booster Tower, and possibly some time afterwards. Attack-wise, it's pretty good- Mario is a hard hitter, and Geno's not bad either, not to mention the fantastic, wonderful Geno Boost. Magic-wise, it's excellent- you're covered for normal magic (Geno), jump and fire (Mario), and thunder and snow (Mallow). However, this team should hit quickly, or you're in trouble- no Toadstool means no Group Hug and no Come Back. The only healing you've got is Mallow's HP Rain and items.
Mario/Geno/Toadstool:This team isn't bad. Mario and Geno are both decent attackers (and Geno Boost. Did I mention Geno Boost). Even though Mallow's element attacks are missing, you still have Mario for the other two and Geno with regular magic. Toadstool provides the healing, and if you don't use the Lazy Shell, can even hold her own attacking alongside the other two.

Mario/Mallow/Toadstool:To be biased, this team is my personal favorite. Bias aside, it can be very useful, particularly against Belome and Culex. Here's why. You've got Mario as your tough attacker, and with proper leveling up either Mallow or the Princess can gain some decent attack power. Magic is completely covered, as is healing. In other words, this team can fight to the death with Regular Attacks and/or effective magic- and then be healed by Toadstool. Here, Toadstool is best off with the Safety Ring- so she can't have a status ailment or instant death when you need her, and the Royal Dress- not the Lazy Shell, because this lowers her attack and magic attack to dangerous levels.