Before we give upgrade tips please remember that each player plays in their own style. Find your own Style of Play. Resistance is about tactical choices, hit & run.
Buildings
Your Headquarters is the most important building. Losing it makes winning very hard and puts player on the back-burner. Try to upgrade its HP & Armour as first priority. 6 upgrades of HP & Armour are considered decent enough.
The most noticeable upgrade is Construction Management System in Construction Yard. That is priority, especially after rank 10+ where you'll see more action on Sea. Max construction area after Resource Production in Supply Depot.
"Insufficient Energy" will be most annoying thing you'll hear in the game. One way to check power sufficiency is to calculate what buildings you can build with HQ level 1 and single Power Plant. Max out Power after Supply.
Maxing out Supply in Supply Depot would be priority. "Resource Production" is important key element which takes you near to Victory. Max it out asap.
Barracks is first military building you need to focus. It gives you infantry for container collection, basic defense and cheap building for building as distraction/ alert against enemy attack. Since Container collection is most important part, unit production time should be priority & to be maxed as you rank up. Thereafter HP/ Armour can be upgraded up to 6 if not above, initially.
Assault Vehicle Factory is your core unit production building. This is building which is priority target for enemy (Especially with Vertex). Unlike Barracks, we suggest to max it out with HP/Armor at 3, thereafter HP/ Armour can be upgraded up to 6, and later up to 9. Unit Production time maxing should be priority if ground army is your preference.
Special Vehicle Factory is your another core unit production building special units. This is building which is priority target for enemy (Especially with Vertex). Unlike Barracks, we suggest to max it out with HP/Armor at 3, thereafter HP/ Armour can be upgraded up to 6, and later up to 9. We recommend to max out Unit Production time for earliest Porcupine.
While Hawks are not as powerful as their counterpart in Confed (Vertex), they remain a strong support unit. Avia Factory is not a priority for many Resistance players since Hawks are usually locked from major upgrades till rank 22. If you love to play air, then we recommend HP/ Armour can be upgraded up to 6, and later up to 9. Unit Production time maxing should be priority if air is your preference. Remember, without Hawks, enemy Vertex & Thors will have free reign over air, so don't ignore Hawks.
Airfield will be first priority for enemy aircraft. Max it's HP & Armour to at least of 6 points of each. Fuel and repair can be upgraded based on your gameplay style.
Navy is loved by many Resistance players. Reasons are that it has fast Caiman for container collection as well as mining, and long range & heavy naval ship, the Alligator which can kill Zeus in 1v1. Shipyard also has, by default, higher HP/ Armour. Since Resistance platforms are not strong against Vikings, we recommend to never ignore Naval. We suggest to max HP before Armor at up to 6, and later to 9.
Nuclear Launcher will be camp-breaker in many scenarios. Given all bad reputation it has received, it'll remain your go to WMD (Weapon of Mass Destruction). While it's usage is still matter of debate, we can suggest to start upgrading it from Gold League onward. Given that it gets destroyed easily by Confed Avia, you will need to focus on maxing out Production Time & Damage first so each launch is quickest and punches maximum damage on enemy base.
Walls are still not the go-to building for Resistance, though it'll help you against rushes. Max out HP before Armour. Keep Armour at at least 3, if not 6.
A well placed Sniper Tower will give you great visibility and warn of any impeding attack. In low ranks, a well upgraded Sniper Tower will hold against rushes. At higher ranks, it'll be a bait for Vertex to attack. Visibility is priority to max out before anything else. Armour should be 80+ if you intend to use it against rushes.
Gun Tower is another building which is not often used since it fails to make impact against enemy attacks given it's cost, unlike its counterpart, the Rocket Tower. However, there will be battles where you'll have enough resources but not enough CP wherein a well placed Gun Tower can provide strong defense against Delta & Fortress rushes as well as Bunker Rushes.
One of the most important building in all towers is Anti Air Tower. Max it out early to avoid enemy Aviation bothering you a lot. HP / Armour to be maxed before anything else since it's the first building to be prioritized in air raids.
Naval Platforms will defend you from stray ships and surprise scouting from a Delta. It's only defense building in sea you have against submarines as well as in case you don't want to go Naval. Max out its damage & range. You can choose to ignore AA if you are doing priority upgrades only.
Units
Riflemen is most critical first unit. It is a scout & container collector. In lower ranks, it'll remain a strong support if not prime attack unit. HP & Armour will make sure that your riflemen will live long to collect containers and to scout. Speed & High Damage will ensure that you will win some rushes.
Grenadiers are anti-vehicle infantry and do their job effectively. You will find them making dent in the enemy even at higher ranks. Upgrade them to protect base from rushes and take out enemy vehicles.
Snipers have love & hate relationship with many players. Some vouch for it's ability to shred enemy infantry, some utilize it's view for scouting, others find it slow & ineffective compared to it's CP of 2. With speed of 60 after upgrades and visual range of 10, it remains one of the strongest scouting units.
Coyote will remain part of battles even at highest ranks due to it's speed, hence strong corresponding scouting potential. At lower ranks, Coyote Rushes are infamous enough. As a scout only unit, you would focus on HP/ Armour so that it survives long enough, whereas as an attack (rush) unit, it is recommended to max it out.
Armadillo is mid tier ground unit which in massed numbers wins many a game. Armadillo rushes are common strategy and would need high HP/ Armor/ Damage. Some people prefer the Mining ability of the Armadillo. A maxed minefield, especially against Infantry & Vehicle, can literally kill an entire attacking army. At higher ranks, its mines kills Hammers, soften up Zeus, delay Torrents giving time to Resistance. Maxing this unit depends on your own battling preference.
Jaguar is the core land Unit of Resistance. It's damage in Siege mode is where battles become easy. In same faction battles, a siege Jaguar will gain easy win over Armadillos. A tip would be to max this unit before entering Gold League.
Another core land Unit of Resistance is the Porcupine. It's only land unit capable of damaging Vertex. Its accompanying machine gun ensures that enemy infantry is not painful and decent armour ensures that it's not a sitting duck for Hammers either. Porcupine is great to pair with Grenadiers or Armadillos, before Jaguars take over the show. We recommend to max this unit before maxing out Jaguar.