List your own picks!
At first, I could only find 3 clear top units, so let's make it the 5 best. That way we'll be exchanging new ideas instead of just stating the obvious.
List your top 5 units and least favorite five units.
1) Wyvern, no explanation necessary.
2) Conscripts (Infantry platoon). They can bubble wrap everything, they can take up board space, they don't much care if they have to snap shoot, and they can hit both T3 and T6 with surprising relative effectiveness, especially when ordered properly. I've seen them shred invisible demons and T6 tyranid MCs. They can excel both in shooting and in the assault. The best part is any time your opponent dedicates firepower to shoot at 3-point models, they're not shooting at anything else more valuable on a per-wound basis.
3) Ministorum priest. Add one of these and your conscripts will take forever to kill. Add two if you're worried about barrage templates.
4) Armored sentinel, because 40 points gets you a bit of long range strength 6 firepower that can hide in cover without getting immobilized and has front armor 12. In the right circumstances, it also makes a great counter-assault unit. There are plenty of assault units that can't touch front armor 12. I'm looking at you, flesh hounds of Khorne, nob-less Ork boyz, and, just to make my point later, Ogryns.
5) Platoon command squad. (Infantry platoon) It's a small, cheap, expendable unit with objective secured and it buffs your other units. What better to hold a backfield objective than one of these, with perhaps sniper rifles or an autocannon? Its small footprint means that it can often hide from sight.
Honorable mention: I want to try forgeworld Sabre defense platforms and manticore platforms, but haven't fielded them yet. Has anyone out there? Are they effective? Also, I can't help enjoying the master of ordnance.
1) Rough riders. Just what you'd expect of cavalry in the year 40,000: they can be easily gunned down before they reach anything. If even necessary.
2) Nork Deddog, Sgt. Kell. Who cares about superior toughness when the unit has majority toughness 3? Who cares about LD8 orders when almost everything already has LD7 or 8? Automatic look out sir will basically protect you from snipers and only snipers, who I don't see much in the meta.
3) Ogryns. They can't scratch AV 12,
4) Leman Russ battle tank (& Demolisher). At ballistic skill 3, it may well miss, and meantime you've paid big points (you better believe you'll regret not taking a dozer blade), some of them for a heavy bolter than always snap fires, many more for AV 14 that strength D will bypass or melta will punch through or which deep strikers or assault units will get around. Good for long range firefights? The Demolisher can't catch Tau, and the battle tank can't punch through riptide armor.
5) Hellhound. Why would you hurry up the field with a vehicle which, in the assault, has rear armor 10 just like most everything else? I can't help comparing it very unfavorably with the helldrake.