He is among the best but his output was inconsistent IMHO, so that he doesn't rank as high as a Beethoven or a Bach.
By inconsistent i don't mean he wrote subpar music along with his good music, since they all did that. It's just that sometimes he falls into mediocrity even in his best work.
Take his opus 80. It's a very fine quartet, but there's just something artificial about it, hard to quantify like that. Same with the first quartet in the opus 44, which is a technical tour de force but also quite dry at times. It's clear that he was attempting to measure up to Beethoven, specifically the opus 59 and the serioso quartet, but, he can't do it. It's a good effort, but Beethoven is just better.
As was Brahms who actually seemed to be inspired by Mendelsshon quite a bit.
