My favourites are the Sky flame and the Night flame.
In my opinion these two flames seem to be very versatile in battle.
If I remember right the effect of the first one is very devastating, while the latter, as we have seen recently, can be deadly if used well by its user.
I really like the concept of the Rain Flames; the whole tranquility factor was a bit confusing at first, but it makes perfect sense now. I love seeing Yamamoto utilize his rain flames in battle; especially against Genkishi which IMO one of the more well-done battles of Reborn.
The idea of Desert Flame seemed cool. But I reckon the Shimon familia are fodder now, so we will never see it in action.
Just to add, least favourite is Swamp. The idea of that seems awful.
And my favourite is Sky or Mist. Sky just looks plain awesome on Tsuna, Xanxus and Byakuran, but Mukuro and Genkishi make Mist look just Inception-like at times lol.
My favourite type of Flame is Sky Flame as it is really usefull and is actually the strongest especially if the user is experienced in it as it can help you predict opponent's movements to some extent and also you can use to some extent other types of Flames and convert them into Sky Flame if I remember it correctly.
Though I also like Lightning, Sun and Earth Flames as they are quite usefull and not only in fighting as the Sun Flame can help you heal yourself and your comrades.
The Sky flame is rare, special and very powerful. It would be my primary choice as it's the most balanced of the flames and strongest.
Sun Flames also if skilled can be super deadly.You can potentially push your body to crazy limits the possibilities are too numerous. Most likely why Reborn is the Greatest Hitman.
Rain can evident weaken any other flame in time and slow people down that's pretty cool. However attack power never seems extra ordinaryly high
Cloud multiples wheter by bearer's flames or absorbing flames from another. It's pretty much allows you to fight kinda by yourself. Takes a Analytic and strategist mind to bring out it's best potential