There were a few solos created later in 4e that did some ridiculous things to overcome action economy (immunity to conditions to make controllers worthless, and multiple off turn actions), but the reality is, I'm with Oofta here. Solo monsters couldn't be used as actual solos. They needed minions at the least, if not support monsters. The very last Solo I ever used was a cakewalk- the Cleric dropped a power on it that made it dazed and vulnerable all for 1 turn, the Fighter hit it with a mark and a utility power that gave it a grand total of -7 to hit anyone other than them, while turtling their defenses, the Barbarian used a 7 [W] daily and the Ranger used an action point to use a daily that hits 3 times, an encounter that hits 2 times, and a bonus action attack to hit one additional time- they got a crit in there somewhere, and they had bracers that added damage if they hit at least twice, plus their hunter's mark.
Something like that. The critter was down to like 25% hit points before it's next turn, and the daze prevented it from using it's off-turn attacks. This isn't really anything new, mind. D&D has always had a problem with this, in my experience. I remember reading The Crystal Shard, with three melee guys going after an ancient white dragon and nodding to myself "yeah, as long as they have the hit points to handle the breath weapon, that's about how it would go", after watching several dragons get massacred.
The only real way for a solo to stand up to a party of adventures is with prep time and devious tricks that aren't usually covered by it's stat block, like the dragon in Dragon Mountain that fools the party with a polymorphed kobold disguised as a red dragon (while it hides, polymorphed into a small bird) to let the party waste their first round only to get ambushed from behind, lol.
Or the dragon in Forge of Fury, which happily uses the underground lake to it's advantage to make it difficult for the party to engage it in melee.
I was using "lair actions" years before 5e, like having a white dragon create it's lair with an icy floor over a frozen lake, uneven slippery terrain, weak points it could break with it's tail, as well as stalactites and stalagmites it could shatter to deal some AoE damage (ice shrapnel), and of course, it's escape route, swimming away to it's secondary lair, an underwater cave, where it keeps it's treasure.
Too many DM's I've seen just grab some monsters and think that will make for a challenging encounter; you need to get inside the monster's head, and force the party to engage it on it's turf or what usually happens is one or two characters take a ton of damage, and then the thing is fuming on hit points before it's third turn.
There are exceptions- monsters that can take players out of the combat with some wacky special ability, but those tend to be pretty miserable for the player who gets basically told "no, you're not allowed to play in this encounter", so I find them hard to use.