If you’ve never been then Phantom of the Opera, still the best.
That is of course based upon you enjoying musicals, generally the Mrs likes them and as it’s your anniversary then something biased towards her will hopefully pay some dividends.
FWIW I’m booking Via Forever for my daughters (in their 20s), that’s the new spice girls one which should be a riot – for them. The Original Abba themed Mama Mia is a laugh and I’ve friends who love We Will Rock You.
If you want other theatre you could try One Man Two Guvnors for comedy, or at the Old Vic “Hedda Gaber” which has got very good reviews (Old Vic is where Kevin Spacey is the artistic director – next to Waterloo Station so not quite a ritzy location as West End)
EDIT: Warhorse as suggested above, that’s a good recommendation