I have used all 3 dantheman mentions. When it comes to spam filtering, all 3 are adequate but none are great.
I liked poco a lot, was a registered user through 2 versions. But the search never felt fast to me. Now I have a mail tree which nears 2Gb in size, so the speed issue is in part a size issue - poco had trouble with the sheer number of folders within folders and all the mail i have, and i had it lock up and crash in normal use several times a week.
Between that and my inability to make it work with imap (both fastmail and dovecot), I had to give up on poco and didnt upgrade. It always breaks my heart a bit when i have to give up on a program I have supported a long time and which does so many things right. I try new updates now and then in hope it gets better for me again.
Going from poco to thunderbird I just couldnt do it. I spent all the time importing all the mboxes (one by one) but I couldnt like it. It did seem to handle the size better, but I didnt use it long enough to really use the search etc. I just had this "what? it doesn't do that?" feeling a too many times - that purely comes from using something for 5 years and missing bits. I guess there's also the part of me that judged it compared to its hype. It is robust though, even with large numbers of messages. Didn't crash, handled imap...
The search felt weaker but I don't remember having to search folder by folder, i think that would have set me off ranting... I'm pretty sure it was able to search in both my imap folders and my poco-imported emails at the same time... but I could be wrong.
I currently use opera mail, have imported about 50% of my old messages, and its search is lovely. It's a quirky tool when it comes to organising, because it doesn't really use folders, just virtual organisation. So if you're the "lots of folders to classify things" type of person it won't be for you. I find that giving up on folders and having one big pot with live filtering and the option of saved searches works really well for me. Backup/export is weak on opera mail though and something that would prevent me from recommending it for many situations.
I still miss poco, and I am still considering trying a few more clients to try to find out a better one.