The ones you had low expectations of, but surprised you?
Mine would be the
SkyRay SA-1,
http://www.manafont.com/product_info.php/sky-ray-sa1-r2-5mode-memory-led-flashlight-1aa-p-5555
and the V-Shark XM-L
Both have way better build quality than I expected, and both have been incredibly reliable, good looking, nice beam. Great value for the money, IMO.
Xeno E03 with XML :bigsmile: Wanted R5 version, bought XML instead. Thought I will sell it promptly, But that floody beam is very usable for close up jobs. Replaced my Ledlenser P5.
Mine has to be the "Jaws Light" The build quality for the money is outstanding. https://budgetlightforum.com/t/-/3735
while generally held in low opinion around here ("OSRAM golden dragon! the horror!") , i have gifted, and given away tens of the uniquefire AA S-1's and have never once had someone say, " oh great, another flashlight.."
never gotten anything but good reviews, people remember the gift long after i have forgotten i gave them one, and they seem to hold up to decent abuse, run without complaint on supermarket batteries and cost 8.30 usd apiece when you order them 10 at a time from DX.
whats not to love?
Heheh…I was going to suggest the same light. Outstanding quality for a $10 torch. In fact, just a couple hours ago I converted mine to an XM-L with a 3A 5-mode driver. Had to rethink my definition of a “pocket rocket” to include this sized torch. 
Pretty much all of my lights might be considered underrated since I'm not interested in two 18650 triple XM-L type of lights or any of the larger lights.
The Ultrafire A30B 2.8 amp 3 mode XML lights I made and gave out to friends. You couldn't pry them out of there hands now. They put out a huge wall of light on medium and draw lots of attention on high.
For an $11 dollar light with $11 dollars in mods they put a lot of very expensive lights to shame.
I guess that by under-rated, I mean lights that did not get great reviews or that were cheaper than most of your other lights but performed better. "Sleepers" is how another member here described them. Ones that surprised you with the quality of build or the beam itself.
It would be difficult for me to put a order to the ones that suprised me but if I had to it would go somthing like this:
#1 Sipik CK35 Recoil. Not mutch buzz about it so I didnt know what to expect. Also under $15
#2 DD group buy UF-2100 3-mode. I read some reviews on it that led me to believe it to be a average light. The one I got draws 3-A (LI-ion) & 3.15 with an IMR on high. Wow!
#3 Sipik/clone SK68. Not realy under-rated, it's an amazing light for the price. Even more so on a 14500
#4 Black Cat HM-01 AAA. Suprised by the amount of light that comes out of something so small akaline or 10440
I don't believe all the above lights are under-rated but they did exceed my expectations.
Ultrafire BJO8A. I hoped it would be what I was looking for but was surprised when it worked out so well.
I forgot about the black Cat AAA. I have one on my keychain, and for $9 I have to say I agree, it is a great light.
DX has them for under $8 now. I think you turned me on to it in a past post. If so thank's.
I think this MXDL is a good buy at less than $6 - very small, takes a 16400 and is very easy to mod.
http://www.tmart.com/3W-200-Lumens-MXDL-Super-Bright-LED-Flashlight-Black_p100653.html