Not caught 2 and 1 yet (working backwards), but, probably...
Series 5
Series 6
Series 4
Junior Apprentice
Series 3
Series 5 was the first I saw - tuned into Ep 1 on a whim when bored, waiting for something-or-other to turn up on iPlayer, and liked it so much I watched Ep 2 on broadcast - so that's probably why I'm more fond of it than the others.
Series 4 and 3 are the only two I didn't catch on broadcast, actually, and while I liked both of 'em, 3 was a little duller than the others, whereas 4 didn't really have many talented candidates. Seemed like a collection of villains. Even by the final, Helene did nothing all series, whereas Alex and Claire were a sort of two-headed monster. Glimmers of competence, but Alex mainly sat around complaining. Lots of fun to watch, but watching people I like doing well is more satisfying than watching the fools rush in.