The one doctrine I feel like saving on is the SpArt doctrine. At best case scenario - how many of those are you going to have? 15? 20 out of 800 brigades you train? Is it worth researching it all just to have some fancy artillery on wheels with your tanks while it can be replaced without much effort by Mot, AC or even Eng? Same goes for Mech/Mot, why not skip them altogether and go with fast Arm tech set (lagging armor to sacrifice defensiveness for speed), combine with Engs and use those as breakthrough and exploits? Hell, you can even go with LArm staying inside the doctrine, if anything can buzz behind enemy line at speed of light with CA bonuses, that'd be those. Do you need that crappy event that reduces your attack value and frontage on defence all that much? How about Guerilla or Delay instead?
1944 Arm can go at 8kph, that's coincidentally also Eng speed and always less than AC speed - even if you tech their armor, not just gun like most people do. Eng morale upgrades with my marines, mountaineers and paratroopers (what major doesn't train those?). AC morale/organization upgrades with my tanks. AC has +2 more Toughness than SpArt and only like 1 point less of SA, comparing 1943 AC to 1942 SpArt. AC techs are difficulty "1" and on 3 year cycle, compare the time it takes to tech them.
Swapping one Arm for Mot means losing 5 points of HA on the one unit that enemy tanks are supposed to fear. It also might mean losing 1kph speed provided you went with a really fast Arm build (1938 Armor in 1944 techs of everything else, feel free to correct me though). IC can't be motivation here - because that's really one thing you have too much of in this game (if not, disregard stupid forum tips and make 1-3 runs in 1936-1937 like you're meant to). If you're that concerned about fuel - why are you using SpArt then instead of 4x cheaper AC or Eng? Supply usage? AC and Eng have half of SpArt's. Here's something to blow your mind, Eng helps on air defence too so next time there's CAS above your tanks, you might wonder why you are paying extra for an inferior brigade. ;-)
The only reason to go with Arm+Mot+SpArt+Whatever is role-playing. But in that case don't call it "best", it didn't even stand next to best...