Magplar is easy, stamplar is not. Stamplar requires you to actively self-heal. You can work around that by wearing Pale Order, but that goes for any class.
I love my magplar for solo and group PvE. Templar's toolkit is very direct and visceral. Stamplar, on the other hand, has nothing going for it right now that I can think of.
I think DK is fun in PvP, not just because it's currently OP, but because Leap is fun. In PvE, while I believe it is or was also meta ... meh. I'd recommend it as a tank, though.
Stamsorc gets my vote as the most effective solo class out of these, but has the caveat that the main skill that allows for that - Crit Surge - is a late unlock. Streak is also a very fun skill, though not one you use in PvE much if your goal is purely to be effective.
the most fun to play in mostly solo PVE
Magplar allows you to stay on the attack with Sweeps, since you heal from Sweeps.
Stamplar requires you to be conscious of your healing or build for ongoing healing (from damage) in other ways, such as via Pale Order or CP. Without those things you'll have to Vigor inbetween Jabs. I find that a pain.
DK is OK, but I find it hard to get good damage out of my stam DK in solo PvE. Like someone else said, you probably want to be a hybrid and you want to have a good rotation. If you have that, I believe DK has been meta in trials. However I always found mag DK overly complicated and less straightforward for self-healing than magplar, magblade or stamsorc. DK is a mixed bag for me. Good tank, decent solo but not as good as other options. Fun in PvP. Leap is great, but if you ever want to solo PvP, nothing beats a nightblade or a sorc IMO. That's despite DK currently being meta and sorc having healing issues.
Stamsorc is as easy or as hard as you want to make it. I haven't played dual-wield in a long time. I play my stamsorc as a Brawler build. Crit Surge + Brawler is the tankiest solo setup of them all, effortlessly beating magplar for sheer survivability in hard content. Ideally you want the Master's 2H weapon from Dragonstar Arena for that, though, and you may want Vicious Ophidian. That said, I'd recommend Vicious Ophidian (stamina) or False God (magicka) for any solo build. Yes, you can achieve much the same effect as those sets with other gear and CP, but they're hard to beat as a package. Brawler, in particular, is sustain hungry, so you want a set like VO to go with that. Stamsorc actually handles the sustain well via Dark Deal, but you don't want to lean on Dark Deal too hard.
In terms of spammable alone you get more easy single target damage out of a templar than you get from a Brawler build, but for AOE damage Master's Brawler builds tie templars or do even better than they do. Of course you can slot other skills for single target on the stamsorc.
In terms of what's fun, that's largely in the eye of the beholder. I can't imagine anyone who doesn't like Leap or Streak, but one thing about the older classes, compared to warden, is that they're more direct. Templar and nightblade both have a very complete situational toolkit, i.e. with executes and gap closers. Magplar damage is very back-loaded. The execute beam does insane damage and can be started at 40% of the target's health, at which point you can back off (to safety) as you execute. I love this aspect of the class as I once executed Balorgh from range in veteran mode while the group was struggling. It's nice to pick out the main target in a mobile fight. As it's a cheap skill you may possibly run the templar beam on a stamplar as well, certainly in PvP, but that would be hybrid territory really.
No one cares much about gap closers in PvE, but Explosive Charge allows you to AOE taunt via the Tormentor set and play as a fake tank. On the stamsorc Stampede with a Tormentor backbar achieves the same trick, while having very real tanking potential.
I find DK kind of complicated. You have to focus on your rotation more to get your AOE done and your healing right, though I'm talking mag or hybrid DK here, I suppose. Can't get away from the hybrid aspect though, as whip consumes both stam and mag. If you (the OP) liked your warden, then you may find a DK fun. I find wardens quite inward looking, rotation heavy and not a lot of fun, but the rythm of that appeals to some people. Templar and nightblade (mag) are terribly straightforward and direct by comparison. I actually find that more fun to play, since skills feel more responsive and you can focus more on the situation than your build, but then again I do have a PvP mindset at heart.
Stamsorc is kind of what you make it. You tend to lean more heavily on weapon skills than on other classes, but boy does the class have passives to get damage out of those skills. Should you play it as a Brawler tank, like me, you'll forgo much of that damage, but you can strike a balance anywhere you like.