I'll just do my top five. Nothing from Super or GT is even worth considering from my POV except maybe the Zamasu and Baby arcs, and I feel like I need to digest them some more.
1. Saiyan arc:
this is the best the series ever got. The plot was solid and had few to no holes. Toriyama's art was it its peak (in my opinion the Cell and Buu arc art is too angular while the pre-23rd Budokai art is too... cartoony, for lack of a better word). Vegeta was a good villain: brutal, arrogant, extremely badass, and an effective foil/evil counterpart to the hero. The henchmen, Raditz and Nappa, had character themselves, and a notable rapport with each other and their boss, rather than just being power levels on legs like Freeza's henchmen. I particularly love Nappa always being shocked at Vegeta's brutality. Goku, Piccolo, and Gohan (especially Piccolo) got some very good development, and all are their most likable in this arc. Even Krillin got his time to shine. Oh, and the fights were of course amazing. There is not one battle in this arc that I did not like. The last fight with everyone ganging up on Vegeta is my favorite sequence in the franchise. I'll let Rocketman sum it up:
There is no super shonen bullshit. No "everything i've got!" last-ditch ultimate move, no "~share your energy~" everybody-contributes malarkey, no suddenly-relevant legends, no self-sacrificing heroism, no miracles. Hell, most of those come up and Vegeta just beats the shit out them.
The main character spends half the fight in a crumpled heap with Vegeta occasionally jumping over and kicking him in the ribs. Mr. Hidden Powaaaaahhhh gets his shit wrecked no matter how mad he gets. All their Super Special Mystical Twaining is useless. The Everybody Helps technique fails completely. Even Oozaru doesn't work - Vegeta cuts off his tail before the beast really damages him. It's only the pure dumb luck that Gohan happened to land on Vegeta that finally brings him down.
It's a beautifully brutal fight that outdoes everything before and after, and it's completely dependent on who Vegeta is: the complete opposite of everything "shonen".
2. Buu arc:
Cipher sums things up rather well, so I'll try not to retread his ground. Fat Buu and Super Buu are both amazing villains. With the latter being cunning, cool, and sadistic, the former being oblivious, childlike, and immature, and both being hilarious and entertaining, they work very well as two sides to the same coin. Super Buu in particular is very multifaceted in personality, being prone to childish outbursts and yet possessing a tactical mind and pragmatic personality. Fat Buu is just very unique in general among DB villains, and, just like his evil half, is loaded with personality. He had a very unique method of 'defeat' as well. Mr. Satan, Piccolo, and Vegeta get some great character arcs. The fights are insane in their uses of unconventional powers and techniques, actually have a sense of scale (my favorite throwaway line in this arc is when Babidi casually mentions that a stray ki blast Goku deflected depopulated 1/10 of the Earth), and are usually very even with few stomp matches.
Overall, DB is a ridiculous series. Even the 'serious' arcs are completely ridiculous, they just pretend they're not. The Buu arc takes that ridiculousness and revels in it, almost becoming a self-parody in the process. This makes the whole thing way more enjoyable, as by contrast I almost felt insulted that the Cell and Freeza arcs tried to pretend that they were anything other than completely absurd.
3. Gets points for having an entertaining villain, a good tone shift, and Yajirobe. Loses points for rehashing plot points, having mostly bland fights, and not giving the supporting cast much of anything to do (except Yajirobe). Still, it was a good transition arc. I give it credit for ushering in a new era.
4. Namek arc:
I'm very deliberately calling this the Namek arc and not the Freeza arc for a reason. I can divide this arc up almost perfectly in terms of what I do and don't like: the first two thirds are okay to good. The last third is crap. The beginning and middle of this arc consists of Gohan and Krillin forming an unlikely alliance with the main villain of the last arc as they use their brains to avoid getting detected. It's great watching Vegeta, Gohan/Krillin, and Freeza's thugs all maneuver around each other. Vegeta, in particular, pulls off several impressive schemes (e.g. hiding the dragon balls from Freeza, attacking Dodoria and Zarbon while they're split up, fighting dirty to defeat monster Zarbon) to get ahead. All this time, the main villain's power is being built up decently well. When Goku arrives, we get a pretty groan-inducing montage of him effortlessly curb-stomping Recoome and Burter in a near-identical manner to the way he took out Nappa, which just feels cheap and repetitive. Then we have the Ginyu debacle. I like that four-way teamwork was required to overcome Ginyu and Jeice, it was a nice subversion of the typical "Goku comes in and kicks everyone's ass" formula that was played straight with Piccolo Daimao, Tao Paipai, and Freeza.
The arc goes severely down hill when Freeza himself enters the fray (see my signature for part of the reason why). He engages in episode after episode/chapter after chapter of fighting with the side characters and Goku's base form. This whole sequence encompassing a third of the arc is pointless, because as he says himself, he's using an absurdly small amount of power in these forms. So we know that none of this matters. We know that, whenever it looks like someone is putting up a fight against Freeza, it's just because he's using like 1% or less of his power. Not that Toriyama even tries to fool us that often, because Freeza is just no-selling everything for 90% of this fight. It's completely bereft of tension or investment. You get the feeling that no one should have even bothered to show up. Especially not poor Piccolo, who is basically a non-entity in this arc. People complain about his Buu arc appearance, but at least there he had a character, a point, a role. Here he just exists to built up and then knocked down to further wank Freeza. The only real stand-outs moments from this part of the arc are Vegeta's final death and Goku's beat down of Freeza the very end.
Speaking of which, while it is the best part of the last third (especially because Freeza can't pull that "I'm only using 50%" bullshit anymore and just gets his ass kicked), is still fairly anticlimactic. As mentioned, Freeza gets his ass kicked. This is only entertaining because he's an awful character, not because watching him get stomped while only landing a 2-3 hits in retaliation is actually interesting (thought it IS actually better than the rest of the fight even on those grounds; at least this is an actual fight and the stronger party is actually taking some damage). On top of that, the scenery was a problem all throughout the arc. Namek, as a setting was basically a testament to Toriyama's laziness. There are multiple suns, so he doesn't have to keep track of the day and night cycle. It's post apocalyptic, so he doesn't have to draw cities or too many people. It's universally identical in terms of scenery, so he doesn't have to draw varied backgrounds. The laziness displayed there is actually pretty impressive. But overall I feel the first half redeems it.
5. Piccolo Daimao arc:
Gets points for having an entertaining villain, a good tone shift, and Yajirobe. Loses points for rehashing plot points, having mostly bland fights, and not giving the supporting cast much of anything to do (except Yajirobe). Still, it was a good transition and I give it credit for ushering in a new era.