agreement of verb

你的問題問得非常好，現引用【牛津　英語用法指南】(Practical English Usage:作者為Michael Swan)的５０６．１條下，書中正有討論 one of the ...+關係從句，相信可以解答你的疑問：
1. one of the ...+ 關係從句
在這個句子裏：She's one of the few women who have climbed Everest（她是幾位登上埃佛勒斯峯的婦女之一），動詞have為複數，因為主詞(who)指的是複數（the few women）。但是，這個句子也包含著She has climbed Everest這樣一句話，而在非正式文體裏，許多人會因此說She's one of the few women who has climbed Everest.嚴格地說，這是不正確的（關係從句中的動詞應與關係從句的主語一致，而不應與主句的主語一致），但在非正式的英語中，這種結構很普遍。
　One of the things that really make/makes me angry people who don't answer letters.（有一件事使我真生氣，那就是人們不答覆信件）
　Alice wa one of the students that were/was late for the lecture.（聴講座有學生遲到，艾麗斯就是其中之一。）

相信看到這樣的講解，你心裏的疑團已經解問！希望幫到你

HAVE is correct, the word HAVE is to describe the best subs (plural), not directly to He.

He was one of those guys who HAVE ever worked here.

2007-10-30 12:16:14 補充：
If you want to use HAS, the sentence should be:He was the best sub that HAS ever worked here.

There is no problem. Because "He was one of the best subs that HAVE ever worked here." , "Have" in fact follows the word "that". In this sentence, "that" represents "He" so it agrees with its nearer antecedent.

That is why we need to put "that" there.

2007-10-30 11:54:07 補充：
Sorry for the may-be confusion, I mean we should use 'Has' instead 'Have' because it follows 'that' which is third person singular.