Omitting YA just for the sake of simplicity
Series seven - Amazing.
Series five - Amazing. Also, the first one I saw, so I see it as the definitive run. Watched S5-S9 and the Young Apprentices live. S4, S3, S2 and S1 were catch-up jobbies.
(Series nine - So far, this is where I'm expecting it to end up. Really digging it.)
Series one - I left this 'til last, and I wish I hadn't. I'd sort of worked back through them over time, and I found the earlier series were a little weaker, more primordial, I didn't feel like they quite hung together as well as later series. Finally, pleasantly surprised to find that S1 was absolutely riveting, start to finish.
Series four - This is the one I think of when someone mentions the Apprentice. I didn't watch it at the time, but I was the most aware of it when it was on; I heard lots of chatter about it at the time, and it had a lot to live up to. The candidates were flappy and the finalists undeserved, but... I already knew that when I went in! Great fun.
Series six - Weird one, this - utter carnage, but played host to some really boring candidates. Chris Farrell, Jamie Lester, Sandeesh Samra, et al., never really earned their place. The candidates who were good telly worked overtime, and I fear Stuart Baggs will probably become the definitive Apprentice candidate, but it's a funny clash of slightly average versus utter mayhem. Not enough good business to rank higher, I didn't feel like I had anybody to really root for except Stuart himself, but a great watch nonetheless.
Series eight - Probably the weakest series I watched live, but still entertaining. Bit of a vibe of "been there, done that." Liked a lot of the candidates as people - Gabby was cool, Ricky was cool, Tom and Nick, great - but it didn't coalesce quite right.
Series three - I feel like I know this one the least. Most eps I've only watched once, never followed it with anyone, and it came slap bang in the middle of an early series catch-up marathon. This placement might be a little unfair, I'm really eager to rewatch it one day, but for now, it's here.
Series two - Ruth's good, Paul's good, but it feels a bit... samey? I'm perfectly happy to watch it, but for what I'd basically consider the breakout series, I don't think it stacks up to any of the others.
That's 7 - 5 - (9) - 1 - 4 - 6 - 8 - 3 - 2. S3 aside (which I think may be unfairly maligned), I seem to have a sort of Star Trek thing going on. Odds vs. Evens.
Young Apprentice, I'd rank thusly...
1. Junior Apprentice
2. Young Apprentice (Ashley)
3. Young Apprentice (Zara)
Junior's hanging somewhere around S4, qualitywise. Ashley's series, just below. Zara's series, somewhere in the S6-S8 fug. Think it shared a lot of the same problems as those two - and like those two, still remained a good watch.