You know, it's actually kind of funny to me that some people loved stage 8 and hated stage 7 because my feelings are quite the opposite. This probably has to do with me as a player in general and my personal development over time, though.
Back in the hail meta in stage 7, offense really ruled over other playstyles. Whether or not you were using hail, every team had to be prepared for it in one way or another. For some teams, this meant carrying Emboar, a huge offensive powerhouse. For others, it meant choosing Duosion over Musharna thanks to Magic Guard and a recovery move not dependent upon weather, Piloswine over other Stealth Rock setters thanks to its ability to check Rotom-Frost and to an extent BlizzSpam, or Rotom-Frost itself over other Choice Scarf users thanks to its then optimal Speed and its typing which kept it from taking hail damage. This was a pretty obvious hassle for more defensively oriented teams. Emboar was incredibly hard to check defensively. Compared to Musharna, Duosion was an inferior check to Fighting-types in general due to its inferior bulk. With offense as the more favorable playstyle, it was then preferable to my personal playstyle.
Stage 8 was really hard for me because my teambuilding was not nearly on the level that it is now. It was much more difficult for me to deviate from the things that I knew worked for me. Musharna, Skuntank, and Sawk (or Gurdurr) found their way onto the majority of my teams because of how consistent they were for me. Musharna in general was a lot harder to take down then compared to now (though it's still not a walk in the park). However, as time has past, I've learned how to support things much better, both defensively and offensively, and it has made it much easier for me to utilize different playstyles. As such, it's quite easy for me to say that stages 9 and 10 are now my favorites, simply because I'm a significantly better player and teambuilder than what I was in previous stages. However, considering myself as a teambuilder now, if I'd have had the same experiences back when I first started playing, I think my favorite metagame would have the same as skylights: stage 7 before hail. I loved to use Cinccino and Emboar, and I miss them much more now than what I did back then (even if the lack of Emboar is super friendly to defensive teams, which I have a soft spot for). Basically, stage 7 had a lot of the diversity that I see and enjoy in the current metagame but with lower Speed tiers, which means that even more things were viable back then compared to now.
To answer the specific questions: from my experiences stage 7 pre-hail was probably the most diverse with hail meta and stage 8 as the least diverse. Keep in mind that this was when I started playing on Smogon, and I played Gothitelle and before on PO. RIP Klinklang, the star of almost all of my PO teams before making the (great) decision to switch to Smogon.
Despite hail meta being one of the least diverse to me, it was still one of my favorites at the time because I personally favored offense. Conversely, stage 8 was a bitch for me because I pigeon-holed myself into using the same things because I couldn't find much else to get the job done as consistently.
General highlights for me are mostly just seeing my progress as a teambuilder (and consequently a battler). Back when I first started out on Smogon, I would have one or two good teams that I would always use. Nowadays, I've got anywhere from fifteen to twenty decent or better teams all at once. Minor highlights would be the gimmicky shit that I've used on the ladder that has worked for me. Special mentions go out to my recent Riolu offense team that has multiple gimmicky sets but still somehow manages to work even outside of the ladder; my sexy Baton Pass team that saw more ragequits than actual victories; and @
Hot N Cold's hax team, packed with such goodies as Swagpard, Sub/Discharge/Air Slash Rotom-S, and ParaFlinch Regirock, that got me to #60 on the ladder.
I won't cover consistent metagame trends because I feel like everyone has covered it well (and I don't want to)!
And I don't really have a favorite metagame. I guess the answer for me would be stages 9 and 10 because those are the very same stages when I grasped the metagame and teambuilding the best. Playing and winning with a variety of teams is obviously a lot more fun than playing and occasionally winning with the same couple of teams over and over again. However, if I could go back to previous stages with this same understanding that I have now, my favorite metagame would undoubtedly be stage 7 pre-hail. There were some top-tier threats, no doubt about it, but there were no over-centralizing playstyles or Pokemon. The Speed tier was much lower then than what it is now, which in and of itself means that the metagame was more diverse then. I like using different things and trying out new playstyles, and I think that if there ever was a stage that I played that best allowed this, it would have for certain been pre-hail stage 7.
Phew, this was a much longer post than what I'd intended, but I kind of really like this concept. It's interesting to look back at BW NU and remember how the metagame has developed and how I've developed as a player. Fuck Musharna before I knew how to handle it without Skuntank; free Klinklang, Cinccino, and Emboar; and as always, @
skylight sucks. :)