Dodgealum, I'd like to correct a few little errors I see being made already ;--))
First concerning 5751's: the 5751's (and 6201's) were originally made for all branches of the military (a "ruggedized" version of the 12AX7 and 12AT7 respectively). And while it's true they have gain curves that are 70% of a 12AX7/12AT7, that difference occurs mostly at THE TOP OF THE CURVE! Most of the curve for the two kinds of tube (12AX7 v. 5751) is identical over the tubes' operational ranges. Think about it; would the military purchase substitute parts that provide only 70% the performance of those they replace? I don't think so.
As for which ones are best, the best place to start reading is (the now ten year old) Joe's Tube Lore at Audio Asylum: http://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html
I agree with most of Joe's conclusions -- certainly tried many of them, but my own preferences are either the Sylvania or the RCA 3-mica blackplates (NOT the grayplates). The consumer version of the Sylvanias were marked "Gold Brand", but most of them carry only military numbers BECAUSE they were for the military!! The Gold PIN Sylvania 3-mica 5751 is NOT a military tube; and, it only came with gray plates! Not a bad tube, but the black plates sound better to me, and there's no need to pay for those fancy gold pins (which you can't see after the tube is installed anyway ;--) AND they ALL have copper plate-support rods (which you CAN see ;--) All the Sylvania and RCA 5751 TMBP (triple mica black plates) have steel pins, and both brands sound about the same to me, (very smooth and grain free compared to ANY 12AX7.) The RCA's are about 25--30% cheaper than the Sylvanias in matched pairs (thanks Joe!) The one 'regular' type 12AX7 I really love (it kick's ass with rock music -- and it's still cheap) is the RCA 17mm long blackplate! Wow!
The 12AT7 v. 6201 story is about the same as for the 5751, with a few variations:
Sylvania 6201 blackplates come in both 2-mica and 3-mica versions. With and without gold pins. Take your pick, they're all good.
RCA 6201 blackplates come in 2 and 3 mica versions as well, IIRC.
Siemens 6201's have excellent bass if for any reason you need a little extra at that end of the frequency spectrum (like bass-shy monitor speakers.)
Valvo blue label with gold pins are considered the best 6201's by some, including me, but not anymore. Read on . . . . .
In my ever so humble opinion ;~)) The absolute, hands down, best 12AT7 (nothing even comes close, no 6201, no anything!) is the GEC A2900; or the military version, the CV6091. They come in a few variations, and I've heard them all. Maybe there are differences, but they are so far ahead of whatever is in second place, it really doesn't matter! They come with 2 or 3 micas, blackplates only, regular or 17mm longplates, pan, square, or halo getters. Pre and post 1970's. The only common/consistent thing about them is their performance and their price: both off the charts; with the former being the reason for the latter ;--) I started collecting them when they were about $85/matched pair on eBay. Now they are hard to find (with good measurements) for much less than $250/pair, whether new (1980's) or vintage (pre 1960's.) I wish I could tell you how I originally stumbled upon them, but that story is lost . . . . . One of the most worthwhile spots for these tubes in a piece of audio equipment, is as drivers for high-performance power tubes (KT88's 6550's) I've used them in my McIntosh 275 to drive NOS Gold Lions, EAT's, and (currently) Shuguang Black Treasure series KT88Z's The effect is a bit like adding octane-booster to your gas ;--) Although you won't realize quite as much improvement with lower transconductance KT88's (like the Russian Gold Lion reissues, Pentas, Electro Harmonix, Svetlanas, etc.) These GEC tubes have so much gain, while being absolutely smooth and quiet, that I'm looking for a tube phonoamp that uses them just to have my mind blown! ;--)
I'm not a fan of Upscale Audio's NOS vintage tube products, primarily because their prices always seem to exceed their tubes' performance, (which is usually average or just below.) Their NOS tubes are generally reliable because they do test them thoroughly; but for best price and highest test measurements, I can do MUCH better on eBay. Plus, UA uses some fictional measurement system which makes it impossible to compare their tubes with tubes measured conventionally -- until you get them home and put them on your own tester . . . . Uh-oh!