There are lot of good drop-ins, different values and different people will offer different advices based on their needs... Based on MY experience, you basically have three very good drop-ins: Malkoff M61(W) for $70, Nailbender XML 2.8A for $45 and Thrunite XML 1.5A for $25...
So now you make what you really appreciate in the light... if you want bomb-proof, Surefire quality dropin, Malkoff M61 is there... might seem a little expensive BUT very versatile, uses XPG but you can use 2 or 3 primaries or 1 or 2 rechargeable cells... 260(240) lumens is very bright and it has great runtimes - 2 hrs on 2xCR123 and few minutes more with 1x18650... installed in MD2 with Hi/Lo is awesome light...
Then you have Nailbender... it's driven rather hard so really no need for 3.1A version, 2.8 is enough... it's so wickedly bright (around 580 lumens on turn on for 2.8A I'm guessing) and draws so much amps on high that you won't be using it more than 3 minutes, thanks to temp you'll feel... and it would shine less than a hour on 1x18650, if it could be left on on high that long... but you will use medium mostly anyway which is about the same brightness as M61 dropin... and you have LOW for pretty long runtimes with night adopted vision...
And there is Thrunite XML... with amp draw of around 1.5A it puts little over 400 lumens on high and pretty much allows you about 90 minutes on high with 1x18650-29 battery... pretty awesome setup in Surefire host with bezel like Ronac Cryos or Solarforce A001... this really seems the best bang for buck if you're going to use 1xLiIon cell, at least IMHO... I think you already made a choice on it... You will have 1 1/2 hours on continuous high with 1x18650-29...
I love all three of them... I'm running M61W in MD2 (new style) that I can extend to get 2x18500 or 3xCR123 batteries and I have hi/lo option... I have Nailbender in Solarforce L2m body on IMR 18350 for ridiculous brightness out of super small host but for short periods and I have C2 with Thrunite XML with Cryos head that I'm pretty sure I can leave on on high for all duration of battery juice, which is like I said about hour and a half for around 400 OTF lumens... not bad...
EDIT:
Malkoff and Nailbender have WARM options and high CRI options where Thrunite has THE MOST NEUTRAL WHITE tint I have seen so far... I absolutely LOVE Thrunite tint but, again, it's me...