Season: at least 15 home runs batting lefty against lefty pitchers.

https://www.baseball-reference.com/

... split may be incomplete due to missing play-by-play data. For all seasons after 1972, the play-by-play data is complete (summary of missing data for pre-1973)...

vs Starter batting splits are for the entire game when the starter was LH or RH. These splits include any subsequent plate appearances against a reliever regardless of their throwing hand. These splits are a proxy for platoon splits for seasons we lack play-by-play data. from GL team pitching splits indicate how batters did against the team and use the batter's overall batting handedness for this split.

________________________

This link contains the source data for lefty batters against lefty pitchers. 

https://stathead.com/tiny/ROgPC

In the Regular Season, from 1901 to 2022, vs LHP as LHB (within Platoon Splits), requiring Home Runs >= 15, sorted by greatest Home Runs.

_______________________

Unfortunately, data is incomplete for these batters: Ruth (1920, 1922), Hurst, Musial, Mize. They probably hit at least 15 home runs against lefty pitchers in those seasons but the actual number is in question and the At Bats (AB) against both lefty and righty pitchers is unknown. These seasons for Ruth are included in the main table:

Ruth 1926 is missing 11 AB and 2 HR.

Ruth 1927 seems complete.

Ruth 1928 is missing 28 AB and 2 HR.

Ruth 1929 is minus only 5 AB against righties.

Sorted on AB/HR against lefties:

Dif:
AB/HR: righty - lefty; positive number means this lefty batter had a better HR rate against lefties: the higher, the better.
HR: HR against lefties - HR against righties: positive number means more HR against lefties.

Comments below the table.

			L	L	L	L	R	R	R	tot	tot	tot	dif	dif	L	L	L	L
Rk	Player	Year	G	HR	AB	AB/HR	HR	AB	AB/HR	HR	AB	AB/HR	AB/HR	HR	BA	OBP	SLG	OPS	Player
1	Barry Bonds	2002	83	21	125	5.95	25	278	11.12	46	403	8.76	5.17	-4	.384	.556	.976	1.532	Barry Bonds
2	Ken Griffey Jr.	1996	94	21	158	7.52	28	387	13.82	49	545	11.12	6.30	-7	.298	.382	.741	1.122	Ken Griffey Jr.
3	Barry Bonds	2003	79	16	124	7.75	29	266	9.17	45	390	8.67	1.42	-13	.363	.509	.790	1.300	Barry Bonds
4	Barry Bonds	2001	95	17	141	8.29	56	335	5.98	73	476	6.52	-2.31	-39	.312	.487	.752	1.239	Barry Bonds
5	Babe Ruth	1927	77	19	158	8.32	41	382	9.32	60	540	9.00	1.00	-22	.323	.476	.728	1.204	Babe Ruth
6	Ken Griffey Jr.	1998	97	21	177	8.43	35	456	13.03	56	633	11.30	4.60	-14	.299	.360	.701	1.061	Ken Griffey Jr.
7	Babe Ruth	1929	54	17	147	8.65	29	346	11.93	46	498	10.83	3.28	-12	.320	.391	.735	1.125	Babe Ruth
8	David Justice	2000	81	15	134	8.93	26	390	15.00	41	524	12.78	6.07	-11	.306	.367	.716	1.084	David Justice
9	Babe Ruth	1928	58	16	150	9.38	36	358	9.94	54	536	9.93	.57	-20	.327	.431	.720	1.151	Babe Ruth
10	Matt Olson	2021	112	22	222	10.09	17	343	20.18	39	565	14.49	10.09	5	.270	.354	.608	.962	Matt Olson
11	Ken Griffey Jr.	1994	69	16	162	10.13	24	271	11.29	40	433	10.83	1.17	-8	.296	.343	.654	.997	Ken Griffey Jr.
12	Joey Gallo	2018	88	15	157	10.47	25	343	13.72	40	500	12.50	3.25	-10	.210	.279	.541	.821	Joey Gallo
13	Cody Bellinger	2019	112	18	193	10.72	29	365	12.59	47	558	11.87	1.86	-11	.280	.386	.596	.982	Cody Bellinger
14	Darryl Strawberry	1988	105	20	216	10.80	19	327	17.21	39	543	13.92	6.41	1	.250	.318	.560	.879	Darryl Strawberry
15	Babe Ruth	1926	78	15	164	10.93	30	320	10.67	47	495	10.53	-.27	-15	.323	.493	.640	1.134	Babe Ruth
16	Shohei Ohtani	2021	104	18	198	11.00	28	339	12.11	46	537	11.67	1.11	-10	.263	.344	.636	.980	Shohei Ohtani
17	Ted Kluszewski	1955	87	20	221	11.05	27	391	14.48	47	612	13.02	3.43	-7	.326	.395	.629	1.024	Ted Kluszewski
18	Rafael Palmeiro	2003	86	15	170	11.33	23	391	17.00	38	561	14.76	5.67	-8	.282	.374	.588	.962	Rafael Palmeiro
19	David Ortiz	2006	107	18	205	11.39	36	353	9.81	54	558	10.33	-1.58	-18	.278	.373	.615	.988	David Ortiz
20	Travis Hafner	2006	86	16	184	11.50	26	270	10.38	42	454	10.81	-1.12	-10	.321	.443	.658	1.100	Travis Hafner
21	Eddie Mathews	1961	78	15	179	11.93	17	393	23.12	32	572	17.88	11.18	-2	.296	.384	.559	.942	Eddie Mathews
22	Curtis Granderson	2011	100	16	191	11.94	25	392	15.68	41	583	14.22	3.74	-9	.272	.347	.597	.944	Curtis Granderson
23	Reggie Jackson	1980	102	19	227	11.95	22	287	13.05	41	514	12.54	1.10	-3	.291	.362	.582	.944	Reggie Jackson
24	Adam Dunn	2012	104	15	183	12.20	26	356	13.69	41	539	13.15	1.49	-11	.191	.302	.465	.767	Adam Dunn
25	Ryan Howard	2006	106	16	197	12.31	42	384	9.14	58	581	10.02	-3.17	-26	.279	.364	.558	.923	Ryan Howard
26	Luis Gonzalez	2001	102	15	186	12.40	42	423	10.07	57	609	10.68	-2.33	-27	.312	.414	.624	1.038	Luis Gonzalez
27	Bryce Harper	2019	97	15	187	12.47	20	386	19.30	35	573	16.37	6.83	-5	.283	.366	.583	.949	Bryce Harper
28	Garret Anderson	2000	88	15	189	12.60	20	458	22.90	35	647	18.49	10.30	-5	.333	.349	.661	1.010	Garret Anderson
29	Ryan Howard	2007	110	16	209	13.06	31	320	10.32	47	529	11.26	-2.74	-15	.225	.333	.493	.826	Ryan Howard
30	Mo Vaughn	1997	106	15	196	13.07	20	331	16.55	35	527	15.06	3.48	-5	.337	.440	.607	1.047	Mo Vaughn
31	Reggie Jackson	1969	94	16	212	13.25	31	337	10.87	47	549	11.68	-2.38	-15	.255	.360	.524	.884	Reggie Jackson
32	Barry Bonds	1990	121	17	240	14.12	16	279	17.44	33	519	15.73	3.32	1	.304	.386	.592	.978	Barry Bonds
33	Darryl Strawberry	1987	114	16	230	14.38	23	302	13.13	39	532	13.64	-1.24	-7	.248	.330	.517	.847	Darryl Strawberry
34	Barry Bonds	1993	107	15	218	14.53	31	375	12.10	46	593	12.89	-2.44	-16	.326	.423	.619	1.042	Barry Bonds
35	Darrell Evans	1973	112	17	248	14.59	24	347	14.46	41	595	14.51	-.13	-7	.307	.428	.589	1.016	Darrell Evans
36	Don Mattingly	1985	109	18	264	14.67	17	388	22.82	35	652	18.63	8.16	1	.288	.328	.568	.896	Don Mattingly
37	Luke Easter	1950	87	16	235	14.69	12	305	25.42	28	540	19.29	10.73	4	.264	.364	.506	.870	Luke Easter
38	Rafael Palmeiro	1997	114	15	225	15.00	23	389	16.91	38	614	16.16	1.91	-8	.213	.275	.440	.715	Rafael Palmeiro
39	Rafael Palmeiro	1998	118	15	230	15.33	28	389	13.89	43	619	14.40	-1.44	-13	.317	.387	.591	.978	Rafael Palmeiro
Rk	Player	Year	G	HR	AB	AB/HR	HR	AB	AB/HR	HR	AB	AB/HR	AB/HR	HR	BA	OBP	SLG	OPS	Player
			L	L	L	L	R	R	R	tot	tot	tot	dif	dif
	max	2021	121	22	264	15.33	56	458	25.42	73	652	19.29	11.18	5	.384	.556	.976	1.532	max
	min	1926	54	15	124	5.95	12	266	5.98	28	390	6.52	-3.17	-39	.191	.275	.440	.715	min
			L	L	L	L	R	R	R	tot	tot	tot	dif	dif	L	L	L	L
Rk	Player	Year	G	HR	AB	AB/HR	HR	AB	AB/HR	HR	AB	AB/HR	AB/HR	HR	BA	OBP	SLG	OPS	Player
	Babe Ruth	1920		15						54	458	8.48			.392	.512	.969	1.481	Babe Ruth
	Babe Ruth	1922		15						35	406	11.60			.308	.410	.798	1.208	Babe Ruth
	Don Hurst	1929		18			13			31	589	19.00		5	.330	.376	.650	1.026	Don Hurst
	Johnny Mize	1947		15						51	586	11.49			.289	.347	.561	.908	Johnny Mize
	Stan Musial	1948		18			19			39	611	15.67		-1	.415	.489	.844	1.333	Stan Musial
	Stan Musial	1949		19			11			36	611	16.97		8	.283	.366	.562	.927	Stan Musial
	Stan Musial	1954		15			20			35	591	16.89		-5	.374	.449	.685	1.134	Stan Musial

The seasons with a lot of incomplete data are separated at the bottom.

Eddie Mathews in 1961 had the biggest difference in home run rates:
against lefties: 11.93
against righties: 23.12
difference: 11.18
total: 17.88

Of the 39 seasons with details, 27 had a better home run rate (AB/HR, lower is better) against lefty pitchers.

Five had more homers against lefties than against righties with these differences:
Matt Olson 5
Luke Easter 4
Barry Bonds 1
Darryl Strawberry 1
Don Mattingly 1