I think my favorite wattage for mercury lamps would have to be 250. I have a 1980's GTE Sylvania 250 watt lamp, and I think it's cool how they cram an arc tube that big into a BT-28 lamp. It also has just the right lumen output for most applications. 12,500 lumens seems like it would be good for many things including streetlights, gyms, malls, etc. I also like 250 watt lamps because the numbers mysteriously work themselves out. 12,500/250=50 lumens per watt. 12,500x2=25,000 lumens. All easy, no oddball numbers.  

Here's the 250 watt lamp running briefly on an H33 ballast. Because it is brand new, the light it puts out is a very bright pink/yellow color.

Nice, but that arctube almost looks MH-ish to me.  But I'm not positive. 
Do you have a 250w ballast? I assume not since it's being tested on a 400w?

No, I do not have a 250 watt ballast. There is one on 1000Bulbs I am looking at though. It's under $40, so it won't break the bank. See here.  

That poor lamp running at almost twice it's rated power. 

My favorite wattage would be a tie between 50 and 100 watts. 50 because it's the most useful wattage for me - I don't have a fixture or application that would make a larger wattage useful. Also because despite ballasts for them being very rare, ballasts still capable of properly running them are VERY common. 

I like 100 because it's the only wattage available in BT-25, which is my favorite HID bulb shape. I hope to get one someday.

I don't think the lamp is any worse for the wear. I hope one day to have a vintage NEMA head or even a full blown street light to put that 250 watt lamp in. All I have now for a streetlight is a 70 watt HPS GE M250-R2.

Guess my favorites for both Mercury and metal halides would be 100,250,and400W.Also like 450W MH Pulse start MH.

I like 100 for the mercs, since it is still fairly common to find westy lifeguards in that wattage.

Have a Sylvania 250W mercury bulb just like the one shown.Really like this lamp.Use it in my 250W Silverliner I got from the site when they changed to LED lamps.It is my favorite-and made in the US.Also have 400W.Like these too-the phosphored 400W is a GE.Use it in a 400W GE cobrahead I got from the plant.I have gotten a strong liking for these lamps-favorites next to my MH lights.

They all are great, but i will choose at a tie between 80 and 125

80 and 125 are moderately low wattages that go into places like small lanterns in footpaths - where the aura of the Mercury light stands out the most

I also have two ken rad mercs and one ITT dating to the mid or late 60s. Never used. Both 250 watts.

I just want to find 50w lamps and gear so I could remote ballast my desk lamp.