I know I'm in the minority but a few of the drive models were pretty versatile IMO. They may not feel exactly like an analog drive pedal but the tube driver in particular always sounded great to me.A lot of the algorithms hold up very well. They're even included on the newer HX units as "Legacy" effects.
Generally, the dirt models aren't well-loved. There are a couple that are fun and don't sound terrible if you take a minute to dial them in.
The filters are fun, but I don't use them too frequently. The synths and pitch models are good to have.
The modulation effects are quite good. I particularly like the MXR flanger models, choruses, DC-2 emulation, and vibrato.
I like some of the reverbs, which are from the Verbzilla (plus the particle). I definitely understand why folks want more updates to the reverbs on the newer units (like Glitz, Ganymede, Plateaux, etc.), but some of the Verbzilla algorithms compare very well to modern mid-priced units.
The delays are from the DL-4 and I like a handful of them. The auto-volume echo is the best I've used and beats out units 3x the price of my M5. The tape and analog emulations are fine, but by no means best-in-class today. They're definitely usable, but it's a bit of a letdown they didn't add the tape and analog modes from the Echo Park (Line 6 was talking about doing this on the forums back in the day).