When I'm constricted to the choice of the "four elements" (air, earth, water, fire) as the foundation for magic, I tend to gravitate towards either air or earth. Sometimes I go for water. I find those three to actually be the more justifiably versatile and overall practical when compared to fire. Granted, I speak to this mostly from the perspective of combative functions.
With fire, you're limited in terms of most respects since it's really just good for creating iterations of fire-based projectiles, with, in my view, no solid defensive or strategic backbone that can't be circumnavigated by the others. In logically run RP, where magic expends energy, as it often must in just about any situation, fire seems the most prone to needing expenditure, since it's not always readily available. Therefore, the actual process of creating fire taking up a portion of ones mana/chi/ether/midichlorian/etc, while even more would have to into actually moving/shaping the fire once it's created. There are some who branch fire out and into controlling smoke, but that's about the extent to which it reaches. Also, a lot of the other techniques that I see people using in connection with fire elements, such as creating fire shields and having DBZ-esque beam deadlocks with fire blasts, just takes a lot of justification and greater suspension of disbelief.
On the other hand, the other elements are often bountiful and almost always present and accessible in some way, shape or form. The raw material already exists for them, meaning that a character can utilize more of their energy to actually make it move/form in the way they need/want it to. Add that to the fact that they are, mostly, found in an array of forms and capable of a wide range of applications, and you've pretty much got a solid trio of powers, any one of which can work ways to outmatch fire.
With air, I'm always quick to justify it as a bridge into use of lightning as sub-element (as much as I love and appreciate the series, the makers of The Last Airbender can suck it on that logic; lightning should have been the trademark ability of a pissed off air bender). I just see air as having plenty of good uses that work great for subtlety. I mean, most of what an air-elemental can do, in terms of conflict situations, can be done in a way that the person they're using it against would barely know it was coming in the first place. One second everything is fine; next second they're being compressed by a sudden increase in air pressure, or being suffocated by sudden shortage of oxygen. At the same time, a good, compressed, gale-force blast of air can knock the wind out of someone pretty well.
With Earth... well in most fantasy settings, there's plenty of it to use, and depending on what qualifies as 'earth,' there's lot of material that's covered under that umbrella. I mainly see earth as a defensive and restricting type of power. Unless you have to deal with another earth manipulating character, you can trap pretty much any problematic adversaries in closed rock coffin, or alter the ground beneath them just enough to make it like quicksand... hell, I once read a tournament battle RP where one player just used an earth manipulator to dig out a concealed pit trap, lure his opponent into it and bury them alive; albeit it helped that the battle was under a restrictive power setting, it was an impressive feat of tact in favor of earth as an elemental affinity.
As for water, while I don't use it quite as often as I used to, it has its merits. The biggest thing that works in favor of water is that a character potentially has three different manners of matter falling under their control. Since water exists in three states as a solid, liquid and gas, it's pretty versatile. Mist veils, whipping someone around with watery tendrils, skewering them or even trapping them, with a good freeze all work pretty well when speaking of combative function. Like air and earth substances, water is something that's easily accessible, existing in most places where characters can actually live and interact and can even be pulled from oddball places, like siphoning the morning dew from the grass and such. That and I do find The Last Airbender's Blood Bending technique as a legitimate branch of water manipulation on non TLAB settings, considering that water must exist in most organisms in sufficient quantity (I've only seen a handful of characters made in my time as an RPers that could justifiably have no water in their bodies, simply through being purely metaphysical beings.
Beyond these elements, I have, of late, gravitated more quickly toward the dual use of electricity & magnetism, to the best of my ability. More often than not though, I have those two off shooting from my much favored use of the concept of chi/ki/prana in characters. Sometimes I just refer to the whole life force concept as being 'ether/aether.' When you get down to it, it's all the same thing given a different name: just raw energy that comprises the makeup of at least the physical realms. In my way of viewing magic, chi/ki etc. is the foundation on which most magic stands. Without the raw energy, no being can perform magical feats. Give a character a more refined control over their base life energy, and you can pretty much grow them in whatever direction you please, power/magic wise.
Of course, I'm pretty flexible when it comes to fandom RPs, and will always work to emulate whatever magical system is in place for whatever canon I might be playing. If I'm doing something in the vein of the RPGs (Final Fantasy, etc) I fit my character's magic skillset to whatever class of character I have them set as; if they're a mage, I'll load them up with spellwork; if they're more a warrior, I'll give them magical abilities that serve to enchant their bodies and weapons to enhance their performance.
Meanwhile, powers that I absolutely abhor (in most cases):
"My character can manipulate time!" For the sheer fact that most people simply leave it at that basic sentence, offering no explanation demonstrative of a sound backing of mechanics behind the facet of existence that their character has control over. With the exception of some fictional settings, 'time' is rarely displayed as having any physical matter; it's a perception of conscious lifeforms. And honestly, it's both more acceptable (in my mind anyways) and simplistic to just define a character as being able to accelerate themselves to a point where 'time' seems to stop than it is to argue that they 'simply stopped time.'
"My character controls Darkness." In the cases of some canon settings this is fine, it severely irks me when people bring it into an original setting that has yet to account for darkness as an 'element.' It's for the same reason I dislike time as a power in most instances. Frequently, when it's used outside of a context in which it has naturally and significantly existed (IE Darkness in the Kingdom Hearts series is defined enough to be somewhat understandable) it often lacks the accompanying explanation of what it is within the universe it's being thrown into. Is it Darkness as in the opposite of light? If so, how is it then tangible? And what then would light be in this context? Or is it simply some obscure for of matter?
I suppose that, in a nutshell, I prefer to use magic that functions on the basis of some sort of understood logic or principle, or otherwise such magic that can be easily given logic or a principle.