I think this is a hard one as it's a totally different game from the "olden days" of Angels and the likes.
T3.6 and before alliances and T4 and after alliances are a very different thing so it's hard to judge who it the best alliance ever on a UK server.
I know some very good players who played in and against Angels and ASBO and but i wasn't playing back in the old days so can't comment first hand on these alliances.
I have played against some decent teams, a Danish outfit came to UK5 when some of AA were playing our one and only supposedly fun non WW server and the Danes absolutely smashed it in a record time which we are still not sure how it was possible in so few days lol! (i do wish we had started that server with the aim to win to get a full idea of how good they might have been vs us but Koch, Hancock and co had a very well organised team of Danish players that I've not come across since or before, i played a COM server with them straight after and the organisation and dedication was insane). They played under Superheroes i think and they get my vote. We were playing as M (Marvel) if my memory servers me right but it was a long time ago.
Other than that I played against a strong Pulse team once when i wasn't in a very well organised alliance and they totally smashed it so i'd have to rank them up very high in their peak.
I had some great times when in HATE but could not class us then as a "Best" alliance as we didn't really play to win the server.s As meme said the server against Smash they played really well and it would have been good to see if they could do it for more than one server but it didn't happen.
Obviously i really enjoy my time leading AA and we do try our hardest to be as organised as we can and improve server on server but we are also in semi retirement now and unfortunately we are past our best, we will of course have one more server though, probably as a smaller group and would be up for other alliances to come and have a scrap. It will probably will be the next starting UK server in September. I hate servers that run through the summer so no more winter starting servers for us, haha.