Craig,
ALL OF THE ABOVE!!! But, of course, being the way I am you all know I'm going to add my 2¢ worth:
A lot depends on the caliber you are using and the energy level you are shooting at. When I am shooting my Theoben with its .22 caliber barrel on I find it loves the JSB Exact 18.3grn pellets the best, but when I switch it over to wearing its .177 caliber set-up, the Crosman Premier Heavy 10.5grn are the best way to go. Of course, when preparing the Crosmans it takes a lot more attention to detail than with the JSB's as they tend to have a lot more slag left in their skirts which requires as certain level of attention to be removed. But, in the end I've found that the extra effort gives obvious results for the .177 caliber as the Crosmans Ballistic Coefficient is the highest of any of the pellets, and their consistency when weighed seems to be pretty decent too.
Now, granted I have only had my Air Arms S400 MPR-FT for a little over a month so haven't played around with testing many of the various pellets out there. But I can tell you this: where the gun is a 12fpe gun the Crosmans didn't fair so well as I had to go down to the Light 7.9grn pellets, which were no where near as accurate as the heavier Air Arms/JSB 8.4grn pellets at 25M.
Another thing that comes into play is what lube, if any, you use to lubricate your pellets. Some will tell you no lubricant is necessary, but I beg to differ. And, here again, it all comes down to the gun/pellet combination. In my Theoben when using the .177 Crosman Premiers I have found that a light coating of the FINISH LINE Teflon Plus DRY TEFLON CHAIN LUBE (the Black Bottle) works best. But when I switch over to the .22, or for my AA MPR-FT I find they like the FINISH LINE WAX LUBRICANT (the Gray Bottle) the best. And yet I believe that Paul Bendix has found that when shooting the exact same 12fpe combination using identical pellets does better with no lube at all. Go figure! I guess finding the right lube/pellet/caliber combination for your gun is a lot like lot testing for a rimfire, other than the cost only being a fraction as much.
And just think, when you go out to shoot that Eley Black, or worse yet, Red Box and blow off 5 or 10 rounds shooting Sighters, or worse yet, a box or two while trying to tune that new barrel, what's it costing you? $2 to $4 dollar on Sighters, and $20 to $40 for tuning? It makes you think! Ah, but when you head out to the range for that first time with your new pellet gun to blow off a full afternoon, all it's going to cost you is maybe $5 or $10 dollars. And that would be for a full 500 rounds of shooting!
Welcome aboard Craig. You're gonna love it!
Dave