Aesaar's list of units that you should always take if able. Not just good units, units that I think should always have a place in your deck if they're available.
Ka-50 AKULA - 150 point USSR prototype attack helicopter. Longest range ATGM in the game (2975m), and carries 12 of them. Also carries 4 Iglas, which are very respectable AA missiles. The gun is defensive only. If it's firing, you're doing something wrong. Biggest downside is that you only get 2.
Ka-52 - 150 point USSR prototype recon helicopter. Only helo in the game to carry antiradar missiles. These outrange most anti-helo AA. Also carries 4 Iglas. The two Kamovs are the only USSR helicopters to have AA missiles.
BMPT - 70 point USSR prototype tanky thing. Do you have a problem with infantry? Send a pair of these. It has the kind of armor you'd expect from a 130 point tank. It has a 30mm autocannon, a 100mm 2430m range HE cannon, and a 30mm grenade launcher. It will slaughter even entrenched infantry squads in seconds.
Su-27PU - 190 point USSR air superiority fighter. It's what you'd expect from a high-end air superiority fighter. What sets it apart from its competition is that it carries 6 medium range (6300m) AMRAAM equivalents rather than 4.
2k22 Tunguska-M - 90 point USSR AA vehicle. This is still probably the best AA unit in the game overall. Radar-guided guns, long-range non-radar missiles. Extremely dependable.
Mi-26 - 120 point USSR cargo helicopter. It's huge. Seriously, it's the size of a C-130. 6000L of supplies. 2 of these is almost as good as a FOB. Keep everything supplied forever. Holy crap vulnerable to AA, so don't use it for front-line resupply. Take some trucks and use this as a mobile FOB.
ADATS - Canadian prototype anti-everything-except-infantry vehicle. The ADATS missile is one of two missiles in the game to be able to target air and ground units. With an AP power of 25, it'll shred tanks from 2800m away, and it's got excellent range against helicopters as well (3125m). Not radar-guided, so nothing to fear from SEAD.
Stormer HVM - British prototype missile tub. The Stormer carries the game's other dual-purpose missile, the Starstreak. Slightly shorter range than the ADATS, and nowhere near the AP power (12), the Stormer makes up for it with accuracy and a ridiculous rate of fire, both of which make it a helicopter shredder. Carries twice the ADATS' ammo load. Also not radar-guided.
Fallschirmjager '90 - West German elite infantry. These guys got toned down a little from what they used to be, but they're still one of the best elite infantry you can get. They're NATO's equivalent to Spetsnaz. They don't have the napalm launcher, but their rifle is much, much better, and they have AT rockets.