Well in my honest opinion you can’t top the FandyFire STL-V6 for under $50 shipped to your door from Deal Xtreme. It’s the best thrower XM-L period for under $50. I love mine.
And if you want the best XM-L throer for $24 shipped to your door, you can’t beat the Small Sun ZY-T13 from Manfont. $ for $ it probably is w/o question even a better deal than the STL-V6.
The Jacob A60 is probably a better deal yet for under $15 shipped, but it’s an XR-E. But man will it throw.
Why? Well I love XM-L throwers, and I love the useable spill they typically offer. Laser beam thowers just aren’t my cup of tea.
Are those lights current regulated?
CNQG sells it for $37 
EDIT: The STL-V2 I mean.
Probably my XM-L C8 modded with a Nanjg 105C driver... Not quite a dedicated thrower, just a decent all-around light that's small enough to carry with minimal inconvenience, but still bright/thowy enough on high.
Although, my 504b with the Manafont 3-mode XM-L drop-in has some added benefit; No crenelations, Floody, for general purpose lighting, still very bright on high, and tailstands.
Tough !
I like the power of my HD2010, but it’s not too pocket friendly size wise.
So I would go with my Keygos M10.
It has a smooth tapering transition from head to body, it’s made well, it’s properly heatsinked, and it’s battery flexible.
It’s bright and it throws well, plus the price is right.
Excellent bang for the buck.
+1
It has to be either a well driven C8 or a well driven P60 & Host.
But that’s not counting shipping.
Another vote for a well-driven C8 XML .
..like XinTD C8 (which has also the best build quality I have ever seen for a C8)
and if how much you spend under $50 doesn’t either, then Bang-4-The Buck I’ll say this one as an EDC. It’s small but IMO it throws and floods in just the right balance, it tail-stands, it has a useable low, it doesn’t get hot, it’s got double heavy-duty o-rings at the pill & one at the tail, deep SMO reflector, XRE, glass lens, and it’s built like a little tank. In fact you can buy 6 of them for nearly under $50.
Granted if you what really want is Jacob A60 kinda power obviously this ain’t it. Personally if I needed to I’d EDC both and that would solve that problem. I’m very impressed with mine.
I have a ultrafire C8 with xml-t6 and stainless steel bezel and SMO reflector, and it’s VERY bright.
bought it for 19.99 dollar here
I think the answer is largely situation dependant. If I have the ability to charge lithiums, keygos m10 no question. If I’m in a primary only situation, minimum 3d Maglite, as I WILL need a club too…. 
yep same here…not a pure thrower but will put light out there…still enough spill for indoors…I have several with the nangj V2 from kd and they’re awesome…
p60 a close second…it will do everything, but does not throw very well and heat sinking is not as good as the C8 style…
Bottom line best.
Have 5 in a dead heat. SE-1, S-Mini, SF L2M with E1320 DI, MIX6 & PA40W. When the dust settled I had to go with the……………………later Edit- I re-read the question & realized the Q was not ‘could’ you pick one, but rather if you ‘had’ to pick one. After mulling over this specific Q with the 5 lights I listed, all things considered, I will go with my S-Mini. Now my other lights ‘feel left out’. Hope your happy .
Keith
Might be a light that is also a charger for cell phones, etc. which has a nice C8 like beam?
http://www.intl-outdoor.com/double-usb-mobile-device-charger-318650-xml-t6-flashlight-p-365.html
http://kaidomain.com/product/details.S020383
After that, however many many non-multipurpose lights is open, I guess.
It's a really hard question to ask guys who have tons of lights , there is no ..."one light scenerio" and by giving a 50$ limit it begs the upper end of the spectrum
Used Nitecore D-10 modded with a nichia or xpgr4 4B
My OEM custom.
I’m with troop in going with the utility of AAs. I like my pa40 a lot, but I don’t have the experience that many of the others have and you have many good suggestions already. I also don’t own many lights (YET).