Here is a fleet list for SIXT. This is not a fully comprehensive list, and if you get a vehicle not on this list, please write below in the comments, and I'll do my best to add it.

Economy (ECAR)
Kia Rio
Hyundai Accent

Compact (CCAR)
Nissan Versa
Hyundai Venue
Kia Soul
Mini Cooper (4-door)

Intermediate (ICAR)
Kia Forte
Hyundai Elantra
Mini Countryman

Standard (SCAR)
VW Jetta
Toyota Corolla
Subaru Impreza

Fullsize (FCAR)
Toyota Camry
VW Passat
Nissan Altima
Subaru Legacy
Dodge Charger

Premium (PCAR)
BMW 2 Series
Mercedes-Benz A-Class
Mercedes-Benz CLA-Class
Infiniti Q50

Midsize Luxury (LCAR)
BMW 3 Series
Mercedes-Benz C-Class sedan
Cadillac CT5
Volvo S60
Volvo V60 (see note below)

Fullsize Luxury (XCAR)
BMW 5 Series
Mercedes-Benz E-Class sedan
Volvo S90
Volvo V90
Volvo V90 Cross Country
Volvo V60 Cross Country (2021 Ride Report)

Special Luxury (XDAR)
BMW 7 Series
Mercedes-Benz S-Class (see note below)

Hybrid Luxury (XCAH)
BMW 5 Series Hybrid

Compact Convertible (CTAR)
Mini Cooper Convertible
VW Beetle Convertible

Standard Convertible (STAR)
Ford Mustang

Midsize Luxury Convertible (LTAR)
Audi A5 cabriolet
Mercedes-Benz C-Class cabriolet
BMW 4 Series cabriolet

Midsize Luxury Coupe (LSAR)
BMW 4 Series coupe

Luxury Convertible (XTAR)
Mercedes-Benz SL-Class cabriolet

Guaranteed Model (XTAJ)
BMW Z4 cabriolet

Guaranteed Model (XTAM)
BMW 8 Series cabriolet

Guaranteed Model (XSAJ)
BMW 8 Series coupe

Intermediate SUV (IFAR)
Toyota RAV4
VW Tiguan
Mazda CX-5
Chevrolet Equinox
GMC Terrain
Ford Escape
Ford Bronco Sport

Standard SUV (SFAR)
Jeep Grand Cherokee
Toyota 4Runner
Chevrolet Blazer
Hyundai Santa Fe
Subaru Outback

Midsize SUV (RFAR)
VW Atlas
GMC Acadia
Chevrolet Traverse
Toyota Highlander
Dodge Durango

Fullsize SUV (FFAR)
Chevrolet Tahoe
Toyota Sequoia
GMC Yukon

Fullsize Elite SUV (GFAR)
Chevrolet Suburban
GMC Yukon XL

Guaranteed Model (LFAE)
Volvo XC40 Recharge

Premium Small SUV (PFAR)
BMW X1
BMW X2
Cadillac XT4
Mercedes-Benz GLA-Class
Mercedes-Benz GLB-Class

Luxury Intermediate SUV (LFAR)
BMW X3
Mercedes-Benz GLC-Class
Cadillac XT5
Volvo XC60

Luxury Standard SUV (XFAR)
BMW X5
Audi Q7
Audi Q8
Cadillac XT6
Volvo XC90
Mercedes-Benz GLE-Class
Range Rover Velar

Fullsize Luxury SUV (XFAZ)
BMW X7
Mercedes-Benz GLS-Class
Cadillac Escalade

Guaranteed Model (XZAN)
Range Rover

Guaranteed Model (XJAN)
Range Rover Sport

Guaranteed Model (XFAN)
Mercedes-Benz G-Class

Guaranteed Model (JFAR)
Jeep Wrangler

Minivan (MVAR)
Chrysler Pacifica
Mercedes-Benz Metris
Dodge Grand Caravan
Kia Sedona
Chrysler Voyager

Hybrid Minivan (MVAH)
Toyota Sienna Hybrid

Passenger Van (FVAR)
Chevrolet Express
GMC Savana

Standard Pickup (SPAR)
Toyota Tacoma

Fullsize Pickup (PPAR)
Toyota Tundra
Ram 1500
Chevrolet Silverado

Notes:
FWD Volvo V60 models are classed as LCAR, however AWD V60's (V60 Cross Country) are classed as XCAR.
It is unknown if any S-Class vehicles actually exist in the fleet. The XDAR fleet is mainly the BMW 7 Series.

This list is taken mainly from the Sixt site, however it is not a fully comprehensive list. This is not sponsored by Sixt in any way. You can help make this list better! Post below what car you got (and what category it was in) and we can make this list more comprehensive!