When I first started playing(at the beginning of D/P) I used a plain mixed team with three sweepers and 3 stallers, without SR(it worked out well, despite that). It is a very simple style, just right for beginners and, if used the right way, also pretty dangerous, imo. Don´t know, why most of the pros out there say that this style is bad...
However after not playing CP for a while(until Platinum), I just went with the flow and built an offensive team. It was fast, but sucked in terms of staying alive lol. I don´t like to EV-train stallers and so I soon built offense only and learned a little. It is necessary to learnthis, because offensive teams are not as easy to build as Mixed teams are.
Nowadays, I use offensive teams, but with bulkier pokemon(not too bulky tough, I want some speed there also^^) and lots and lots of U-Turn and complementing resistances, because it´s fun to mess around with that move and slowly getting to see the opponent´s team. I don´t really know the definition of "semistall" or "bulky offense"(can someone tell me?) but I don´t think my team fits either of them.
Simply put, in my opinion, there is no "best team structure". Each and everyone should just use what HE/SHE is able to play with and THAT´s finally the personal best team structure. As GeorgeSlayer said:
I personally think that there is no single best team structure;all of them have their pros and cons.Finding one's individual playstyle so as to make playing more enjoyful and at the same time effective is what matters the most as I see it.
But to fit in a little:
Stall seems to be the worst style out there, imo. I mean, who had the idea of putting 6 strong stallers on one team? They usually can´t do much damage and have the problem that is there since the very beginning: 6 mons simply aren´t enough to counter/wall everything. Also, it´s annoying, how stall teams stretch battles endlessly....