As they said, there really is no best for most tanks. MH and T5s are tried and true lighting options that we know work in almost any situation. LEDs are almost there, but there have been several accounts of people having coral (SPS most commonly) not survive the switch in the long term. There are NUMEROUS accounts of people bleaching coral due to the high, concentrated PAR values when they switch, but that's just poor planning. It is not a fault in technology. The problems I mentioned are described on RC by a few people and they mention poor coloration, little or no growth and sometimes coral death after a switch. Again, this is mostly (from what I've read) an SPS problem and not a very wide spread one. I know there are several people on this board running LEDs with plenty of success.
You have to decide what you want out of your lighting. You can probably get the cheapest lighting by buying some used T5 or MH gear from someone and using it. New MH and T5 lighting isn't terribly expensive, but it can get up there. LEDs hit you with a larger up front cost, but their operational life is much longer than that of MH or T5s, so not only do you save on the electric bill, but you save on bulb replacement costs. Another feather in the LED hat is the lack of heat transfer into the water. MH, and to some extent T5s, tend to cause heat problems on small tanks. It can be avoided with proper installation and use of fans in many cases, but LEDs eleminate the problem all together. If I had to sum it up, this is what I'd say are the biggest thoughts in my mind when determining lighting needs:
MH:
Pros - tried and true, I love the 250w Radium bulb, retrofit kits are relatively cheap and the pendents look nice
Cons - individual bulb replacement cost, heat, high wattage, no dusk/dawn effect w/o supplements
T5:
Pro - More bulbs means you can tweak your color spectrum more than MH, often fixtures are lower profile than MH, can cut back on bulb change cost impact by working out a schedule to do one at a time, less heat than MH, lower power than MH
Cons: no shimmer, bulb replacement at once can be expensive, perfect color can be difficult to attain (trial and error), bulb length is related to fixture length and can be problematic for some tanks aesthetically
LED:
Pro: Low energy use, low heat transfer, blues really make colors pop, fixtures are modern looking and sleek,
Cons: A few corals seem they may have issues with LEDs, lighting can seem cold and synthetic, cost is high unless DIY (but it is falling), LEDs make a properly lit tank look dim to the eye
As people experiment with LEDs, I think the problems will be dealt with and they will eventually be the dominant light in the trade. I think the issues with the SPS may come from the fact that LEDs are very narrow in spectrum and the fixtures used are just missing something. As people tinker and LED tech moves forward I bet this is solved. The cold/synthetic color can be worked around with different LED colors mixed in, and this is the strong point of DIY or the Radion lights newly released.
I'm still running MH, but I'm open minded about LEDs