I agree with reviewers who comment that the LPO sometimes lacks the polish of other orchestras, and that Tennstedt sometimes lacks the kind of concentration that Solti can bring (for better *and* worse), but more than any other conductor Tennstedt's Mahler sounds fresher, clearer, and less emotionally contrived. Tennstedt takes a spacious view of Mahler, and his slightly slower tempos often bring a clarity that I find most welcome. It is true that in Nos. 2 and 5, the intensity could be greater, and in No. 9 the more direct view seems a bit too understated (if you're used to Bernstein, like myself). However, Nos. 3, 4, 6, and 8 are absolutely fantastic! Powerful and lyrical, these accounts reveal a depth of musical composition and emotional passion that is tremendously moving. The other symphonies are also excellent, if not outstanding (these symphonies are all very well represented on CD, with a variety of excellent accounts). No. 1 sets the tone right away, and is particularly notable for the less moulded style than Mahler fans may be used to. There are tempo changes that sometimes don't quite seem natural (like in the last mvmt of No. 1), but this did not overly concern me. No. 7 could perhaps be a bit more seductive in the 'night music', but the first and last movements are well conceived and sound strong and convincing.
I own cycles by Solti and Bernstein, with recordings by Walter, Klemperer, Haitink, Abbado, Rattle, Tilson Thomas, and others. The Tennstedt cycle may be the best introduction to Mahler if not the best 'all time'. This is because of his relatively direct, honest, and natural approach to music that so often invites unique personal interpretations. Encountering Bernstein, Solti, Rattle, and others after hearing Tennstedt first may be the wisest (musically) way to go.