Either 6.1 or 6.2.
Thinking on losing, quite a lot of things stacked together really
I lost the Andron/Wraith terror mission in the US which boosted panic up a ton and was broke constantly so I couldn't build a new base there to reduce UFO activity which cost me it as both North and South America were unable to be calmed down
2nd base I had in Asia was essentially worthless because I didn't have enough to make two hangers or two interceptors and by that time most UFOs then were either Observers I couldn't catch unless I upgraded the engines using the resources I couldn't shoot down to get or they had a Defender escort who I couldn't take on alone
The 8 infiltrators reduced a lot of cash flow because I had to spend all my points on removing them or any that popped up, when most of the time I'd put those points into getting a starter fund for a new base or aircraft to help bootstrap my panic management, which indirectly led to me losing the air war and probably the campaign
Funding was much lower than I anticipated, combined with multiple mission losses and the costs of losing so many people or MARS units, just couldn't afford much so I couldn't hire who I needed or build myself up
Had a total loss on a Cleaner Convoy which wiped out my MARS, all my A Team and their gear meaning I lost a huge amount of resources, there were so many cleaners that it made it quite hard to not be overwhelmed even when I had the jump on them and managed to kill or supress quite a few on the first turn
I also had a really bad abduction because the map was quite small so the enemies were close together and the landing zone was a kill zone, the artic map where you're surrounded by fencing and there's cargo containers up ahead, you can't really advance without having your sides exposed, I've had bad missions there before but this felt worse since I had less troops and more aliens
A couple times where a unlucky dodge or dogfight led to a Angel getting downed due to the 150% UFO damage, leaving me unable to tackle the Phase 2 UFO wave and panic outbreaks, infiltrations or get resources from crashes
On average, the amount of troops and missions I lost were way higher than most campaigns due to I feel like having less soldiers and more enemies who have better aim. Losing armour on death, making new weapons, rebuilding MARS units and hiring up new troops really can drain your funding that it makes you unable to put it towards anything else, and when considering funding is lower and the amount of infiltrators is higher, your ability to spend Op points on a emergency reserve of cash is really strongly reduced, especially when you're using them to also reduce panic in countries you can't reach yet.